Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
“GNU Free Documentation License”.

1 About the GNU Coding Standards

The GNU Coding Standards were written by Richard Stallman and other GNU
Project volunteers. Their purpose is to make the GNU system clean,
consistent, and easy to install. This document can also be read as a
guide to writing portable, robust and reliable programs. It focuses on
programs written in C, but many of the rules and principles are useful
even if you write in another programming language. The rules often
state reasons for writing in a certain way.

If you did not obtain this file directly from the GNU project and
recently, please check for a newer version. You can get the GNU
Coding Standards from the GNU web server in many
different formats, including the Texinfo source, PDF, HTML, DVI, plain
text, and more, at: http://www.gnu.org/prep/standards/.

If you are maintaining an official GNU package, in addition to this
document, please read and follow the GNU maintainer information
(see Contents in Information for Maintainers of GNU
Software).

Please send corrections or suggestions for this document to
bug-standards@gnu.org. If you make a suggestion, please
include a suggested new wording for it, to help us consider the
suggestion efficiently. We prefer a context diff to the Texinfo
source, but if that’s difficult for you, you can make a context diff
for some other version of this document, or propose it in any way that
makes it clear. The source repository for this document can be found
at http://savannah.gnu.org/projects/gnustandards.

These standards cover the minimum of what is important when writing a
GNU package. Likely, the need for additional standards will come up.
Sometimes, you might suggest that such standards be added to this
document. If you think your standards would be generally useful, please
do suggest them.

You should also set standards for your package on many questions not
addressed or not firmly specified here. The most important point is to
be self-consistent—try to stick to the conventions you pick, and try
to document them as much as possible. That way, your program will be
more maintainable by others.

2.1 Referring to Proprietary Programs

Don’t in any circumstances refer to Unix source code for or during
your work on GNU! (Or to any other proprietary programs.)

If you have a vague recollection of the internals of a Unix program,
this does not absolutely mean you can’t write an imitation of it, but
do try to organize the imitation internally along different lines,
because this is likely to make the details of the Unix version
irrelevant and dissimilar to your results.

For example, Unix utilities were generally optimized to minimize
memory use; if you go for speed instead, your program will be very
different. You could keep the entire input file in memory and scan it
there instead of using stdio. Use a smarter algorithm discovered more
recently than the Unix program. Eliminate use of temporary files. Do
it in one pass instead of two (we did this in the assembler).

Or, on the contrary, emphasize simplicity instead of speed. For some
applications, the speed of today’s computers makes simpler algorithms
adequate.

Or go for generality. For example, Unix programs often have static
tables or fixed-size strings, which make for arbitrary limits; use
dynamic allocation instead. Make sure your program handles NULs and
other funny characters in the input files. Add a programming language
for extensibility and write part of the program in that language.

Or turn some parts of the program into independently usable libraries.
Or use a simple garbage collector instead of tracking precisely when
to free memory, or use a new GNU facility such as obstacks.

2.2 Accepting Contributions

If the program you are working on is copyrighted by the Free Software
Foundation, then when someone else sends you a piece of code to add to
the program, we need legal papers to use it—just as we asked you to
sign papers initially. Each person who makes a nontrivial
contribution to a program must sign some sort of legal papers in order
for us to have clear title to the program; the main author alone is not
enough.

So, before adding in any contributions from other people, please tell
us, so we can arrange to get the papers. Then wait until we tell you
that we have received the signed papers, before you actually use the
contribution.

This applies both before you release the program and afterward. If
you receive diffs to fix a bug, and they make significant changes, we
need legal papers for that change.

This also applies to comments and documentation files. For copyright
law, comments and code are just text. Copyright applies to all kinds of
text, so we need legal papers for all kinds.

We know it is frustrating to ask for legal papers; it’s frustrating for
us as well. But if you don’t wait, you are going out on a limb—for
example, what if the contributor’s employer won’t sign a disclaimer?
You might have to take that code out again!

You don’t need papers for changes of a few lines here or there, since
they are not significant for copyright purposes. Also, you don’t need
papers if all you get from the suggestion is some ideas, not actual code
which you use. For example, if someone sent you one implementation, but
you write a different implementation of the same idea, you don’t need to
get papers.

The very worst thing is if you forget to tell us about the other
contributor. We could be very embarrassed in court some day as a
result.

We have more detailed advice for maintainers of GNU packages. If you
have reached the stage of maintaining a GNU program (whether released
or not), please take a look: see Legal Matters in Information for GNU Maintainers.

2.3 Trademarks

Please do not include any trademark acknowledgements in GNU software
packages or documentation.

Trademark acknowledgements are the statements that such-and-such is a
trademark of so-and-so. The GNU Project has no objection to the basic
idea of trademarks, but these acknowledgements feel like kowtowing,
and there is no legal requirement for them, so we don’t use them.

What is legally required, as regards other people’s trademarks, is to
avoid using them in ways which a reader might reasonably understand as
naming or labeling our own programs or activities. For example, since
“Objective C” is (or at least was) a trademark, we made sure to say
that we provide a “compiler for the Objective C language” rather
than an “Objective C compiler”. The latter would have been meant as
a shorter way of saying the former, but it does not explicitly state
the relationship, so it could be misinterpreted as using “Objective
C” as a label for the compiler rather than for the language.

Please don’t use “win” as an abbreviation for Microsoft Windows in
GNU software or documentation. In hacker terminology, calling
something a “win” is a form of praise. If you wish to praise
Microsoft Windows when speaking on your own, by all means do so, but
not in GNU software. Usually we write the name “Windows” in full,
but when brevity is very important (as in file names and sometimes
symbol names), we abbreviate it to “w”. For instance, the files and
functions in Emacs that deal with Windows start with ‘w32’.

3.1 Which Languages to Use

When you want to use a language that gets compiled and runs at high
speed, the best language to use is C. C++ is ok too, but please don’t
make heavy use of templates. So is Java, if you compile it.

When highest efficiency is not required, other languages commonly used
in the free software community, such as Lisp, Scheme, Python, Ruby, and
Java, are OK too. Scheme, as implemented by GNU Guile, plays a
particular role in the GNU System: it is the preferred language to
extend programs written in C/C++, and also a fine language for a wide
range of applications. The more GNU components use Guile and Scheme,
the more users are able to extend and combine them (see The Emacs
Thesis in GNU Guile Reference Manual).

Many programs are designed to be extensible: they include an interpreter
for a language that is higher level than C. Often much of the program
is written in that language, too. The Emacs editor pioneered this
technique.

The standard extensibility interpreter for GNU software is Guile
(http://www.gnu.org/software/guile/), which implements the
language Scheme (an especially clean and simple dialect of Lisp).
Guile also includes bindings for GTK+/GNOME, making it practical to
write modern GUI functionality within Guile. We don’t reject programs
written in other “scripting languages” such as Perl and Python, but
using Guile is the path that will lead to overall consistency of the
GNU system.

3.2 Compatibility with Other Implementations

With occasional exceptions, utility programs and libraries for GNU
should be upward compatible with those in Berkeley Unix, and upward
compatible with Standard C if Standard C specifies their
behavior, and upward compatible with POSIX if POSIX specifies
their behavior.

When these standards conflict, it is useful to offer compatibility
modes for each of them.

Standard C and POSIX prohibit many kinds of extensions. Feel
free to make the extensions anyway, and include a ‘--ansi’,
‘--posix’, or ‘--compatible’ option to turn them off.
However, if the extension has a significant chance of breaking any real
programs or scripts, then it is not really upward compatible. So you
should try to redesign its interface to make it upward compatible.

Many GNU programs suppress extensions that conflict with POSIX if the
environment variable POSIXLY_CORRECT is defined (even if it is
defined with a null value). Please make your program recognize this
variable if appropriate.

When a feature is used only by users (not by programs or command
files), and it is done poorly in Unix, feel free to replace it
completely with something totally different and better. (For example,
vi is replaced with Emacs.) But it is nice to offer a compatible
feature as well. (There is a free vi clone, so we offer it.)

Additional useful features are welcome regardless of whether
there is any precedent for them.

3.3 Using Non-standard Features

Many GNU facilities that already exist support a number of convenient
extensions over the comparable Unix facilities. Whether to use these
extensions in implementing your program is a difficult question.

On the one hand, using the extensions can make a cleaner program.
On the other hand, people will not be able to build the program
unless the other GNU tools are available. This might cause the
program to work on fewer kinds of machines.

With some extensions, it might be easy to provide both alternatives.
For example, you can define functions with a “keyword” INLINE
and define that as a macro to expand into either inline or
nothing, depending on the compiler.

In general, perhaps it is best not to use the extensions if you can
straightforwardly do without them, but to use the extensions if they
are a big improvement.

An exception to this rule are the large, established programs (such as
Emacs) which run on a great variety of systems. Using GNU extensions in
such programs would make many users unhappy, so we don’t do that.

Another exception is for programs that are used as part of compilation:
anything that must be compiled with other compilers in order to
bootstrap the GNU compilation facilities. If these require the GNU
compiler, then no one can compile them without having them installed
already. That would be extremely troublesome in certain cases.

3.4 Standard C and Pre-Standard C

1989 Standard C is widespread enough now that it is ok to use its
features in programs. There is one exception: do not ever use the
“trigraph” feature of Standard C.

The 1999 and 2011 editions of Standard C are not fully supported
on all platforms. If you aim to support compilation by
compilers other than GCC, you should not require these C
features in your programs. It is ok to use these features
conditionally when the compiler supports them.

If your program is only meant to compile with GCC, then you can
use these features if GCC supports them, when they give substantial
benefit.

However, it is easy to support pre-standard compilers in most programs,
so if you know how to do that, feel free.

To support pre-standard C, instead of writing function definitions in
standard prototype form,

int
foo (int x, int y)
…

write the definition in pre-standard style like this,

int
foo (x, y)
int x, y;
…

and use a separate declaration to specify the argument prototype:

int foo (int, int);

You need such a declaration anyway, in a header file, to get the benefit
of prototypes in all the files where the function is called. And once
you have the declaration, you normally lose nothing by writing the
function definition in the pre-standard style.

This technique does not work for integer types narrower than int.
If you think of an argument as being of a type narrower than int,
declare it as int instead.

There are a few special cases where this technique is hard to use. For
example, if a function argument needs to hold the system type
dev_t, you run into trouble, because dev_t is shorter than
int on some machines; but you cannot use int instead,
because dev_t is wider than int on some machines. There
is no type you can safely use on all machines in a non-standard
definition. The only way to support non-standard C and pass such an
argument is to check the width of dev_t using Autoconf and choose
the argument type accordingly. This may not be worth the trouble.

In order to support pre-standard compilers that do not recognize
prototypes, you may want to use a preprocessor macro like this:

3.5 Conditional Compilation

When supporting configuration options already known when building your
program we prefer using if (... ) over conditional compilation,
as in the former case the compiler is able to perform more extensive
checking of all possible code paths.

For example, please write

if (HAS_FOO)
...
else
...

instead of:

#ifdef HAS_FOO
...
#else
...
#endif

A modern compiler such as GCC will generate exactly the same code in
both cases, and we have been using similar techniques with good success
in several projects. Of course, the former method assumes that
HAS_FOO is defined as either 0 or 1.

While this is not a silver bullet solving all portability problems,
and is not always appropriate, following this policy would have saved
GCC developers many hours, or even days, per year.

In the case of function-like macros like REVERSIBLE_CC_MODE in
GCC which cannot be simply used in if (...) statements, there is
an easy workaround. Simply introduce another macro
HAS_REVERSIBLE_CC_MODE as in the following example:

4.1 Non-GNU Standards

The GNU Project regards standards published by other organizations as
suggestions, not orders. We consider those standards, but we do not
“obey” them. In developing a GNU program, you should implement
an outside standard’s specifications when that makes the GNU system
better overall in an objective sense. When it doesn’t, you shouldn’t.

In most cases, following published standards is convenient for
users—it means that their programs or scripts will work more
portably. For instance, GCC implements nearly all the features of
Standard C as specified by that standard. C program developers would
be unhappy if it did not. And GNU utilities mostly follow
specifications of POSIX.2; shell script writers and users would be
unhappy if our programs were incompatible.

But we do not follow either of these specifications rigidly, and there
are specific points on which we decided not to follow them, so as to
make the GNU system better for users.

For instance, Standard C says that nearly all extensions to C are
prohibited. How silly! GCC implements many extensions, some of which
were later adopted as part of the standard. If you want these
constructs to give an error message as “required” by the standard,
you must specify ‘--pedantic’, which was implemented only so that
we can say “GCC is a 100% implementation of the standard”, not
because there is any reason to actually use it.

POSIX.2 specifies that ‘df’ and ‘du’ must output sizes by
default in units of 512 bytes. What users want is units of 1k, so
that is what we do by default. If you want the ridiculous behavior
“required” by POSIX, you must set the environment variable
‘POSIXLY_CORRECT’ (which was originally going to be named
‘POSIX_ME_HARDER’).

GNU utilities also depart from the letter of the POSIX.2 specification
when they support long-named command-line options, and intermixing
options with ordinary arguments. This minor incompatibility with
POSIX is never a problem in practice, and it is very useful.

In particular, don’t reject a new feature, or remove an old one,
merely because a standard says it is “forbidden” or “deprecated”.

4.2 Writing Robust Programs

Avoid arbitrary limits on the length or number of any data
structure, including file names, lines, files, and symbols, by allocating
all data structures dynamically. In most Unix utilities, “long lines
are silently truncated”. This is not acceptable in a GNU utility.

Utilities reading files should not drop NUL characters, or any other
nonprinting characters including those with codes above 0177.
The only sensible exceptions would be utilities specifically intended
for interface to certain types of terminals or printers that can’t
handle those characters. Whenever possible, try to make programs work
properly with sequences of bytes that represent multibyte characters;
UTF-8 is the most important.

Check every system call for an error return, unless you know you wish
to ignore errors. Include the system error text (from perror,
strerror, or equivalent) in every error message
resulting from a failing system call, as well as the name of the file
if any and the name of the utility. Just “cannot open foo.c” or
“stat failed” is not sufficient.

Check every call to malloc or realloc to see if it
returned zero. Check realloc even if you are making the block
smaller; in a system that rounds block sizes to a power of 2,
realloc may get a different block if you ask for less space.

In Unix, realloc can destroy the storage block if it returns
zero. GNU realloc does not have this bug: if it fails, the
original block is unchanged. Feel free to assume the bug is fixed. If
you wish to run your program on Unix, and wish to avoid lossage in this
case, you can use the GNU malloc.

You must expect free to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling free.

If malloc fails in a noninteractive program, make that a fatal
error. In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop. This allows the user to kill other processes to free up
virtual memory, and then try the command again.

Use getopt_long to decode arguments, unless the argument syntax
makes this unreasonable.

When static storage is to be written in during program execution, use
explicit C code to initialize it. Reserve C initialized declarations
for data that will not be changed.

Try to avoid low-level interfaces to obscure Unix data structures (such
as file directories, utmp, or the layout of kernel memory), since these
are less likely to work compatibly. If you need to find all the files
in a directory, use readdir or some other high-level interface.
These are supported compatibly by GNU.

The preferred signal handling facilities are the BSD variant of
signal, and the POSIX sigaction function; the
alternative USG signal interface is an inferior design.

Nowadays, using the POSIX signal functions may be the easiest way
to make a program portable. If you use signal, then on GNU/Linux
systems running GNU libc version 1, you should include
bsd/signal.h instead of signal.h, so as to get BSD
behavior. It is up to you whether to support systems where
signal has only the USG behavior, or give up on them.

In error checks that detect “impossible” conditions, just abort.
There is usually no point in printing any message. These checks
indicate the existence of bugs. Whoever wants to fix the bugs will have
to read the source code and run a debugger. So explain the problem with
comments in the source. The relevant data will be in variables, which
are easy to examine with the debugger, so there is no point moving them
elsewhere.

Do not use a count of errors as the exit status for a program.
That does not work, because exit status values are limited to 8
bits (0 through 255). A single run of the program might have 256
errors; if you try to return 256 as the exit status, the parent process
will see 0 as the status, and it will appear that the program succeeded.

If you make temporary files, check the TMPDIR environment
variable; if that variable is defined, use the specified directory
instead of /tmp.

In addition, be aware that there is a possible security problem when
creating temporary files in world-writable directories. In C, you can
avoid this problem by creating temporary files in this manner:

fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);

or by using the mkstemps function from Gnulib
(see mkstemps in Gnulib).

In bash, use set -C (long name noclobber) to avoid this
problem. In addition, the mktemp utility is a more general
solution for creating temporary files from shell scripts
(see mktemp invocation in GNU Coreutils).

4.3 Library Behavior

Try to make library functions reentrant. If they need to do dynamic
storage allocation, at least try to avoid any nonreentrancy aside from
that of malloc itself.

Here are certain name conventions for libraries, to avoid name
conflicts.

Choose a name prefix for the library, more than two characters long.
All external function and variable names should start with this
prefix. In addition, there should only be one of these in any given
library member. This usually means putting each one in a separate
source file.

An exception can be made when two external symbols are always used
together, so that no reasonable program could use one without the
other; then they can both go in the same file.

External symbols that are not documented entry points for the user
should have names beginning with ‘_’. The ‘_’ should be
followed by the chosen name prefix for the library, to prevent
collisions with other libraries. These can go in the same files with
user entry points if you like.

Static functions and variables can be used as you like and need not
fit any naming convention.

Line numbers should start from 1 at the beginning of the file, and
column numbers should start from 1 at the beginning of the line.
(Both of these conventions are chosen for compatibility.) Calculate
column numbers assuming that space and all ASCII printing characters
have equal width, and assuming tab stops every 8 columns. For
non-ASCII characters, Unicode character widths should be used when in
a UTF-8 locale; GNU libc and GNU gnulib provide suitable
wcwidth functions.

The error message can also give both the starting and ending positions
of the erroneous text. There are several formats so that you can
avoid redundant information such as a duplicate line number.
Here are the possible formats:

Error messages from other noninteractive programs should look like this:

program:sourcefile:lineno: message

when there is an appropriate source file, or like this:

program: message

when there is no relevant source file.

If you want to mention the column number, use this format:

program:sourcefile:lineno:column: message

In an interactive program (one that is reading commands from a
terminal), it is better not to include the program name in an error
message. The place to indicate which program is running is in the
prompt or with the screen layout. (When the same program runs with
input from a source other than a terminal, it is not interactive and
would do best to print error messages using the noninteractive style.)

The string message should not begin with a capital letter when
it follows a program name and/or file name, because that isn’t the
beginning of a sentence. (The sentence conceptually starts at the
beginning of the line.) Also, it should not end with a period.

Error messages from interactive programs, and other messages such as
usage messages, should start with a capital letter. But they should not
end with a period.

4.5 Standards for Interfaces Generally

Please don’t make the behavior of a utility depend on the name used
to invoke it. It is useful sometimes to make a link to a utility
with a different name, and that should not change what it does.

Instead, use a run time option or a compilation switch or both to
select among the alternate behaviors. You can also build two versions
of the program, with different names and different default behaviors.

Likewise, please don’t make the behavior of a command-line program
depend on the type of output device it gets as standard output or
standard input. Device independence is an important principle of the
system’s design; do not compromise it merely to save someone from
typing an option now and then. (Variation in error message syntax
when using a terminal is ok, because that is a side issue that people
do not depend on.)

If you think one behavior is most useful when the output is to a
terminal, and another is most useful when the output is a file or a
pipe, then it is usually best to make the default behavior the one
that is useful with output to a terminal, and have an option for the
other behavior. You can also build two different versions of the
program with different names.

There is an exception for programs whose output in certain cases is
binary data. Sending such output to a terminal is useless and can
cause trouble. If such a program normally sends its output to stdout,
it should detect, in these cases, when the output is a terminal and
give an error message instead. The -f option should override
this exception, thus permitting the output to go to the terminal.

Compatibility requires certain programs to depend on the type of output
device. It would be disastrous if ls or sh did not do so
in the way all users expect. In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type. For example, we provide a dir program much
like ls except that its default output format is always
multi-column format.

4.6 Standards for Graphical Interfaces

When you write a program that provides a graphical user interface,
please make it work with the X Window System, using the GTK+ toolkit
or the GNUstep toolkit, unless the functionality specifically requires
some alternative (for example, “displaying jpeg images while in
console mode”).

In addition, please provide a command-line interface to control the
functionality. (In many cases, the graphical user interface can be a
separate program which invokes the command-line program.) This is
so that the same jobs can be done from scripts.

Please also consider providing a D-bus interface for use from other
running programs, such as within GNOME. (GNOME used to use CORBA
for this, but that is being phased out.) In addition, consider
providing a library interface (for use from C), and perhaps a
keyboard-driven console interface (for use by users from console
mode). Once you are doing the work to provide the functionality and
the graphical interface, these won’t be much extra work.

4.7 Standards for Command Line Interfaces

It is a good idea to follow the POSIX guidelines for the
command-line options of a program. The easiest way to do this is to use
getopt to parse them. Note that the GNU version of getopt
will normally permit options anywhere among the arguments unless the
special argument ‘--’ is used. This is not what POSIX
specifies; it is a GNU extension.

Please define long-named options that are equivalent to the
single-letter Unix-style options. We hope to make GNU more user
friendly this way. This is easy to do with the GNU function
getopt_long.

One of the advantages of long-named options is that they can be
consistent from program to program. For example, users should be able
to expect the “verbose” option of any GNU program which has one, to be
spelled precisely ‘--verbose’. To achieve this uniformity, look at
the table of common long-option names when you choose the option names
for your program (see Option Table).

It is usually a good idea for file names given as ordinary arguments to
be input files only; any output files would be specified using options
(preferably ‘-o’ or ‘--output’). Even if you allow an output
file name as an ordinary argument for compatibility, try to provide an
option as another way to specify it. This will lead to more consistency
among GNU utilities, and fewer idiosyncrasies for users to remember.

All programs should support two standard options: ‘--version’
and ‘--help’. CGI programs should accept these as command-line
options, and also if given as the PATH_INFO; for instance,
visiting ‘http://example.org/p.cgi/--help’ in a browser should
output the same information as invoking ‘p.cgi --help’ from the
command line.

4.7.1 --version

The standard --version option should direct the program to
print information about its name, version, origin and legal status,
all on standard output, and then exit successfully. Other options and
arguments should be ignored once this is seen, and the program should
not perform its normal function.

The first line is meant to be easy for a program to parse; the version
number proper starts after the last space. In addition, it contains
the canonical name for this program, in this format:

GNU Emacs 19.30

The program’s name should be a constant string; don’t compute it
from argv[0]. The idea is to state the standard or canonical
name for the program, not its file name. There are other ways to find
out the precise file name where a command is found in PATH.

If the program is a subsidiary part of a larger package, mention the
package name in parentheses, like this:

emacsserver (GNU Emacs) 19.30

If the package has a version number which is different from this
program’s version number, you can mention the package version number
just before the close-parenthesis.

If you need to mention the version numbers of libraries which
are distributed separately from the package which contains this program,
you can do so by printing an additional line of version info for each
library you want to mention. Use the same format for these lines as for
the first line.

Please do not mention all of the libraries that the program uses “just
for completeness”—that would produce a lot of unhelpful clutter.
Please mention library version numbers only if you find in practice that
they are very important to you in debugging.

The following line, after the version number line or lines, should be a
copyright notice. If more than one copyright notice is called for, put
each on a separate line.

Next should follow a line stating the license, preferably using one of
abbreviations below, and a brief statement that the program is free
software, and that users are free to copy and change it. Also mention
that there is no warranty, to the extent permitted by law. See
recommended wording below.

It is ok to finish the output with a list of the major authors of the
program, as a way of giving credit.

Here’s an example of output that follows these rules:

GNU hello 2.3
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

You should adapt this to your program, of course, filling in the proper
year, copyright holder, name of program, and the references to
distribution terms, and changing the rest of the wording as necessary.

This copyright notice only needs to mention the most recent year in
which changes were made—there’s no need to list the years for previous
versions’ changes. You don’t have to mention the name of the program in
these notices, if that is inconvenient, since it appeared in the first
line. (The rules are different for copyright notices in source files;
see Copyright Notices in Information for GNU Maintainers.)

Translations of the above lines must preserve the validity of the
copyright notices (see Internationalization). If the translation’s
character set supports it, the ‘(C)’ should be replaced with the
copyright symbol, as follows:

Write the word “Copyright” exactly like that, in English. Do not
translate it into another language. International treaties recognize
the English word “Copyright”; translations into other languages do not
have legal significance.

Finally, here is the table of our suggested license abbreviations.
Any abbreviation can be followed by ‘vversion[+]’, meaning
that particular version, or later versions with the ‘+’, as shown
above. In the case of a GNU license, always indicate the permitted
versions in this way.

In the case of exceptions for extra permissions with the GPL, we use
‘/’ for a separator; the version number can follow the license
abbreviation as usual, as in the examples below.

4.7.2 --help

The standard --help option should output brief documentation
for how to invoke the program, on standard output, then exit
successfully. Other options and arguments should be ignored once this
is seen, and the program should not perform its normal function.

Near the end of the ‘--help’ option’s output, please place lines
giving the email address for bug reports, the package’s home page
(normally ‘http://www.gnu.org/software/pkg’, and the
general page for help using GNU programs. The format should be like this:

Report bugs to: mailing-addresspkg home page: <http://www.gnu.org/software/pkg/>
General help using GNU software: <http://www.gnu.org/gethelp/>

4.8 Standards for Dynamic Plug-in Interfaces

Another aspect of keeping free programs free is encouraging
development of free plug-ins, and discouraging development of
proprietary plug-ins. Many GNU programs will not have anything like
plug-ins at all, but those that do should follow these
practices.

First, the general plug-in architecture design should closely tie the
plug-in to the original code, such that the plug-in and the base
program are parts of one extended program. For GCC, for example,
plug-ins receive and modify GCC’s internal data structures, and so
clearly form an extended program with the base GCC.

Second, you should require plug-in developers to affirm that their
plug-ins are released under an appropriate license. This should be
enforced with a simple programmatic check. For GCC, again for
example, a plug-in must define the global symbol
plugin_is_GPL_compatible, thus asserting that the plug-in is
released under a GPL-compatible license (see Plugins in GCC Internals).

By adding this check to your program you are not creating a new legal
requirement. The GPL itself requires plug-ins to be free software,
licensed compatibly. As long as you have followed the first rule above
to keep plug-ins closely tied to your original program, the GPL and AGPL
already require those plug-ins to be released under a compatible
license. The symbol definition in the plug-in—or whatever equivalent
works best in your program—makes it harder for anyone who might
distribute proprietary plug-ins to legally defend themselves. If a case
about this got to court, we can point to that symbol as evidence that
the plug-in developer understood that the license had this requirement.

4.9 Table of Long Options

Here is a table of long options used by GNU programs. It is surely
incomplete, but we aim to list all the options that a new program might
want to be compatible with. If you use names not already in the table,
please send bug-standards@gnu.org a list of them, with their
meanings, so we can update the table.

‘after-date’

‘-N’ in tar.

‘all’

‘-a’ in du, ls, nm, stty, uname,
and unexpand.

‘all-text’

‘-a’ in diff.

‘almost-all’

‘-A’ in ls.

‘append’

‘-a’ in etags, tee, time;
‘-r’ in tar.

‘archive’

‘-a’ in cp.

‘archive-name’

‘-n’ in shar.

‘arglength’

‘-l’ in m4.

‘ascii’

‘-a’ in diff.

‘assign’

‘-v’ in gawk.

‘assume-new’

‘-W’ in make.

‘assume-old’

‘-o’ in make.

‘auto-check’

‘-a’ in recode.

‘auto-pager’

‘-a’ in wdiff.

‘auto-reference’

‘-A’ in ptx.

‘avoid-wraps’

‘-n’ in wdiff.

‘background’

For server programs, run in the background.

‘backward-search’

‘-B’ in ctags.

‘basename’

‘-f’ in shar.

‘batch’

Used in GDB.

‘baud’

Used in GDB.

‘before’

‘-b’ in tac.

‘binary’

‘-b’ in cpio and diff.

‘bits-per-code’

‘-b’ in shar.

‘block-size’

Used in cpio and tar.

‘blocks’

‘-b’ in head and tail.

‘break-file’

‘-b’ in ptx.

‘brief’

Used in various programs to make output shorter.

‘bytes’

‘-c’ in head, split, and tail.

‘c++’

‘-C’ in etags.

‘catenate’

‘-A’ in tar.

‘cd’

Used in various programs to specify the directory to use.

‘changes’

‘-c’ in chgrp and chown.

‘classify’

‘-F’ in ls.

‘colons’

‘-c’ in recode.

‘command’

‘-c’ in su;
‘-x’ in GDB.

‘compare’

‘-d’ in tar.

‘compat’

Used in gawk.

‘compress’

‘-Z’ in tar and shar.

‘concatenate’

‘-A’ in tar.

‘confirmation’

‘-w’ in tar.

‘context’

Used in diff.

‘copyleft’

‘-W copyleft’ in gawk.

‘copyright’

‘-C’ in ptx, recode, and wdiff;
‘-W copyright’ in gawk.

‘core’

Used in GDB.

‘count’

‘-q’ in who.

‘count-links’

‘-l’ in du.

‘create’

Used in tar and cpio.

‘cut-mark’

‘-c’ in shar.

‘cxref’

‘-x’ in ctags.

‘date’

‘-d’ in touch.

‘debug’

‘-d’ in make and m4;
‘-t’ in Bison.

‘define’

‘-D’ in m4.

‘defines’

‘-d’ in Bison and ctags.

‘delete’

‘-D’ in tar.

‘dereference’

‘-L’ in chgrp, chown, cpio, du,
ls, and tar.

‘dereference-args’

‘-D’ in du.

‘device’

Specify an I/O device (special file name).

‘diacritics’

‘-d’ in recode.

‘dictionary-order’

‘-d’ in look.

‘diff’

‘-d’ in tar.

‘digits’

‘-n’ in csplit.

‘directory’

Specify the directory to use, in various programs. In ls, it
means to show directories themselves rather than their contents. In
rm and ln, it means to not treat links to directories
specially.

‘discard-all’

‘-x’ in strip.

‘discard-locals’

‘-X’ in strip.

‘dry-run’

‘-n’ in make.

‘ed’

‘-e’ in diff.

‘elide-empty-files’

‘-z’ in csplit.

‘end-delete’

‘-x’ in wdiff.

‘end-insert’

‘-z’ in wdiff.

‘entire-new-file’

‘-N’ in diff.

‘environment-overrides’

‘-e’ in make.

‘eof’

‘-e’ in xargs.

‘epoch’

Used in GDB.

‘error-limit’

Used in makeinfo.

‘error-output’

‘-o’ in m4.

‘escape’

‘-b’ in ls.

‘exclude-from’

‘-X’ in tar.

‘exec’

Used in GDB.

‘exit’

‘-x’ in xargs.

‘exit-0’

‘-e’ in unshar.

‘expand-tabs’

‘-t’ in diff.

‘expression’

‘-e’ in sed.

‘extern-only’

‘-g’ in nm.

‘extract’

‘-i’ in cpio;
‘-x’ in tar.

‘faces’

‘-f’ in finger.

‘fast’

‘-f’ in su.

‘fatal-warnings’

‘-E’ in m4.

‘file’

‘-f’ in gawk, info, make, mt,
sed, and tar.

‘field-separator’

‘-F’ in gawk.

‘file-prefix’

‘-b’ in Bison.

‘file-type’

‘-F’ in ls.

‘files-from’

‘-T’ in tar.

‘fill-column’

Used in makeinfo.

‘flag-truncation’

‘-F’ in ptx.

‘fixed-output-files’

‘-y’ in Bison.

‘follow’

‘-f’ in tail.

‘footnote-style’

Used in makeinfo.

‘force’

‘-f’ in cp, ln, mv, and rm.

‘force-prefix’

‘-F’ in shar.

‘foreground’

For server programs, run in the foreground;
in other words, don’t do anything special to run the server
in the background.

‘format’

Used in ls, time, and ptx.

‘freeze-state’

‘-F’ in m4.

‘fullname’

Used in GDB.

‘gap-size’

‘-g’ in ptx.

‘get’

‘-x’ in tar.

‘graphic’

‘-i’ in ul.

‘graphics’

‘-g’ in recode.

‘group’

‘-g’ in install.

‘gzip’

‘-z’ in tar and shar.

‘hashsize’

‘-H’ in m4.

‘header’

‘-h’ in objdump and recode

‘heading’

‘-H’ in who.

‘help’

Used to ask for brief usage information.

‘here-delimiter’

‘-d’ in shar.

‘hide-control-chars’

‘-q’ in ls.

‘html’

In makeinfo, output HTML.

‘idle’

‘-u’ in who.

‘ifdef’

‘-D’ in diff.

‘ignore’

‘-I’ in ls;
‘-x’ in recode.

‘ignore-all-space’

‘-w’ in diff.

‘ignore-backups’

‘-B’ in ls.

‘ignore-blank-lines’

‘-B’ in diff.

‘ignore-case’

‘-f’ in look and ptx;
‘-i’ in diff and wdiff.

‘ignore-errors’

‘-i’ in make.

‘ignore-file’

‘-i’ in ptx.

‘ignore-indentation’

‘-I’ in etags.

‘ignore-init-file’

‘-f’ in Oleo.

‘ignore-interrupts’

‘-i’ in tee.

‘ignore-matching-lines’

‘-I’ in diff.

‘ignore-space-change’

‘-b’ in diff.

‘ignore-zeros’

‘-i’ in tar.

‘include’

‘-i’ in etags;
‘-I’ in m4.

‘include-dir’

‘-I’ in make.

‘incremental’

‘-G’ in tar.

‘info’

‘-i’, ‘-l’, and ‘-m’ in Finger.

‘init-file’

In some programs, specify the name of the file to read as the user’s
init file.

‘initial’

‘-i’ in expand.

‘initial-tab’

‘-T’ in diff.

‘inode’

‘-i’ in ls.

‘interactive’

‘-i’ in cp, ln, mv, rm;
‘-e’ in m4;
‘-p’ in xargs;
‘-w’ in tar.

‘intermix-type’

‘-p’ in shar.

‘iso-8601’

Used in date

‘jobs’

‘-j’ in make.

‘just-print’

‘-n’ in make.

‘keep-going’

‘-k’ in make.

‘keep-files’

‘-k’ in csplit.

‘kilobytes’

‘-k’ in du and ls.

‘language’

‘-l’ in etags.

‘less-mode’

‘-l’ in wdiff.

‘level-for-gzip’

‘-g’ in shar.

‘line-bytes’

‘-C’ in split.

‘lines’

Used in split, head, and tail.

‘link’

‘-l’ in cpio.

‘lint’

‘lint-old’

Used in gawk.

‘list’

‘-t’ in cpio;
‘-l’ in recode.

‘list’

‘-t’ in tar.

‘literal’

‘-N’ in ls.

‘load-average’

‘-l’ in make.

‘login’

Used in su.

‘machine’

Used in uname.

‘macro-name’

‘-M’ in ptx.

‘mail’

‘-m’ in hello and uname.

‘make-directories’

‘-d’ in cpio.

‘makefile’

‘-f’ in make.

‘mapped’

Used in GDB.

‘max-args’

‘-n’ in xargs.

‘max-chars’

‘-n’ in xargs.

‘max-lines’

‘-l’ in xargs.

‘max-load’

‘-l’ in make.

‘max-procs’

‘-P’ in xargs.

‘mesg’

‘-T’ in who.

‘message’

‘-T’ in who.

‘minimal’

‘-d’ in diff.

‘mixed-uuencode’

‘-M’ in shar.

‘mode’

‘-m’ in install, mkdir, and mkfifo.

‘modification-time’

‘-m’ in tar.

‘multi-volume’

‘-M’ in tar.

‘name-prefix’

‘-a’ in Bison.

‘nesting-limit’

‘-L’ in m4.

‘net-headers’

‘-a’ in shar.

‘new-file’

‘-W’ in make.

‘no-builtin-rules’

‘-r’ in make.

‘no-character-count’

‘-w’ in shar.

‘no-check-existing’

‘-x’ in shar.

‘no-common’

‘-3’ in wdiff.

‘no-create’

‘-c’ in touch.

‘no-defines’

‘-D’ in etags.

‘no-deleted’

‘-1’ in wdiff.

‘no-dereference’

‘-d’ in cp.

‘no-inserted’

‘-2’ in wdiff.

‘no-keep-going’

‘-S’ in make.

‘no-lines’

‘-l’ in Bison.

‘no-piping’

‘-P’ in shar.

‘no-prof’

‘-e’ in gprof.

‘no-regex’

‘-R’ in etags.

‘no-sort’

‘-p’ in nm.

‘no-splash’

Don’t print a startup splash screen.

‘no-split’

Used in makeinfo.

‘no-static’

‘-a’ in gprof.

‘no-time’

‘-E’ in gprof.

‘no-timestamp’

‘-m’ in shar.

‘no-validate’

Used in makeinfo.

‘no-wait’

Used in emacsclient.

‘no-warn’

Used in various programs to inhibit warnings.

‘node’

‘-n’ in info.

‘nodename’

‘-n’ in uname.

‘nonmatching’

‘-f’ in cpio.

‘nstuff’

‘-n’ in objdump.

‘null’

‘-0’ in xargs.

‘number’

‘-n’ in cat.

‘number-nonblank’

‘-b’ in cat.

‘numeric-sort’

‘-n’ in nm.

‘numeric-uid-gid’

‘-n’ in cpio and ls.

‘nx’

Used in GDB.

‘old-archive’

‘-o’ in tar.

‘old-file’

‘-o’ in make.

‘one-file-system’

‘-l’ in tar, cp, and du.

‘only-file’

‘-o’ in ptx.

‘only-prof’

‘-f’ in gprof.

‘only-time’

‘-F’ in gprof.

‘options’

‘-o’ in getopt, fdlist, fdmount,
fdmountd, and fdumount.

‘output’

In various programs, specify the output file name.

‘output-prefix’

‘-o’ in shar.

‘override’

‘-o’ in rm.

‘overwrite’

‘-c’ in unshar.

‘owner’

‘-o’ in install.

‘paginate’

‘-l’ in diff.

‘paragraph-indent’

Used in makeinfo.

‘parents’

‘-p’ in mkdir and rmdir.

‘pass-all’

‘-p’ in ul.

‘pass-through’

‘-p’ in cpio.

‘port’

‘-P’ in finger.

‘portability’

‘-c’ in cpio and tar.

‘posix’

Used in gawk.

‘prefix-builtins’

‘-P’ in m4.

‘prefix’

‘-f’ in csplit.

‘preserve’

Used in tar and cp.

‘preserve-environment’

‘-p’ in su.

‘preserve-modification-time’

‘-m’ in cpio.

‘preserve-order’

‘-s’ in tar.

‘preserve-permissions’

‘-p’ in tar.

‘print’

‘-l’ in diff.

‘print-chars’

‘-L’ in cmp.

‘print-data-base’

‘-p’ in make.

‘print-directory’

‘-w’ in make.

‘print-file-name’

‘-o’ in nm.

‘print-symdefs’

‘-s’ in nm.

‘printer’

‘-p’ in wdiff.

‘prompt’

‘-p’ in ed.

‘proxy’

Specify an HTTP proxy.

‘query-user’

‘-X’ in shar.

‘question’

‘-q’ in make.

‘quiet’

Used in many programs to inhibit the usual output. Every
program accepting ‘--quiet’ should accept ‘--silent’ as a
synonym.

‘quiet-unshar’

‘-Q’ in shar

‘quote-name’

‘-Q’ in ls.

‘rcs’

‘-n’ in diff.

‘re-interval’

Used in gawk.

‘read-full-blocks’

‘-B’ in tar.

‘readnow’

Used in GDB.

‘recon’

‘-n’ in make.

‘record-number’

‘-R’ in tar.

‘recursive’

Used in chgrp, chown, cp, ls, diff,
and rm.

‘reference’

‘-r’ in touch.

‘references’

‘-r’ in ptx.

‘regex’

‘-r’ in tac and etags.

‘release’

‘-r’ in uname.

‘reload-state’

‘-R’ in m4.

‘relocation’

‘-r’ in objdump.

‘rename’

‘-r’ in cpio.

‘replace’

‘-i’ in xargs.

‘report-identical-files’

‘-s’ in diff.

‘reset-access-time’

‘-a’ in cpio.

‘reverse’

‘-r’ in ls and nm.

‘reversed-ed’

‘-f’ in diff.

‘right-side-defs’

‘-R’ in ptx.

‘same-order’

‘-s’ in tar.

‘same-permissions’

‘-p’ in tar.

‘save’

‘-g’ in stty.

‘se’

Used in GDB.

‘sentence-regexp’

‘-S’ in ptx.

‘separate-dirs’

‘-S’ in du.

‘separator’

‘-s’ in tac.

‘sequence’

Used by recode to chose files or pipes for sequencing passes.

‘shell’

‘-s’ in su.

‘show-all’

‘-A’ in cat.

‘show-c-function’

‘-p’ in diff.

‘show-ends’

‘-E’ in cat.

‘show-function-line’

‘-F’ in diff.

‘show-tabs’

‘-T’ in cat.

‘silent’

Used in many programs to inhibit the usual output.
Every program accepting
‘--silent’ should accept ‘--quiet’ as a synonym.

‘size’

‘-s’ in ls.

‘socket’

Specify a file descriptor for a network server to use for its socket,
instead of opening and binding a new socket. This provides a way to
run, in a non-privileged process, a server that normally needs a
reserved port number.

‘sort’

Used in ls.

‘source’

‘-W source’ in gawk.

‘sparse’

‘-S’ in tar.

‘speed-large-files’

‘-H’ in diff.

‘split-at’

‘-E’ in unshar.

‘split-size-limit’

‘-L’ in shar.

‘squeeze-blank’

‘-s’ in cat.

‘start-delete’

‘-w’ in wdiff.

‘start-insert’

‘-y’ in wdiff.

‘starting-file’

Used in tar and diff to specify which file within
a directory to start processing with.

4.10 OID Allocations

The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
GNU Project (thanks to Sergey Poznyakoff). These are used for SNMP,
LDAP, X.509 certificates, and so on. The web site
http://www.alvestrand.no/objectid has a (voluntary) listing of
many OID assignments.

If you need a new slot for your GNU package, write
maintainers@gnu.org. Here is a list of arcs currently
assigned:

4.11 Memory Usage

If a program typically uses just a few meg of memory, don’t bother making any
effort to reduce memory usage. For example, if it is impractical for
other reasons to operate on files more than a few meg long, it is
reasonable to read entire input files into memory to operate on them.

However, for programs such as cat or tail, that can
usefully operate on very large files, it is important to avoid using a
technique that would artificially limit the size of files it can handle.
If a program works by lines and could be applied to arbitrary
user-supplied input files, it should keep only a line in memory, because
this is not very hard and users will want to be able to operate on input
files that are bigger than will fit in memory all at once.

If your program creates complicated data structures, just make them in
memory and give a fatal error if malloc returns zero.

Memory analysis tools such as valgrind can be useful, but
don’t complicate a program merely to avoid their false alarms. For
example, if memory is used until just before a process exits, don’t
free it simply to silence such a tool.

4.12 File Usage

Programs should be prepared to operate when /usr and /etc
are read-only file systems. Thus, if the program manages log files,
lock files, backup files, score files, or any other files which are
modified for internal purposes, these files should not be stored in
/usr or /etc.

There are two exceptions. /etc is used to store system
configuration information; it is reasonable for a program to modify
files in /etc when its job is to update the system configuration.
Also, if the user explicitly asks to modify one file in a directory, it
is reasonable for the program to store other files in the same
directory.

5.1 Formatting Your Source Code

Please keep the length of source lines to 79 characters or less, for
maximum readability in the widest range of environments.

It is important to put the open-brace that starts the body of a C
function in column one, so that they will start a defun. Several
tools look for open-braces in column one to find the beginnings of C
functions. These tools will not work on code not formatted that way.

Avoid putting open-brace, open-parenthesis or open-bracket in column
one when they are inside a function, so that they won’t start a defun.
The open-brace that starts a struct body can go in column one
if you find it useful to treat that definition as a defun.

It is also important for function definitions to start the name of the
function in column one. This helps people to search for function
definitions, and may also help certain tools recognize them. Thus,
using Standard C syntax, the format is this:

static char *
concat (char *s1, char *s2)
{
…
}

or, if you want to use traditional C syntax, format the definition like
this:

For struct and enum types, likewise put the braces in
column one, unless the whole contents fits on one line:

struct foo
{
int a, b;
}

or

struct foo { int a, b; }

The rest of this section gives our recommendations for other aspects of
C formatting style, which is also the default style of the indent
program in version 1.2 and newer. It corresponds to the options

We don’t think of these recommendations as requirements, because it
causes no problems for users if two different programs have different
formatting styles.

But whatever style you use, please use it consistently, since a mixture
of styles within one program tends to look ugly. If you are
contributing changes to an existing program, please follow the style of
that program.

Please use formfeed characters (control-L) to divide the program into
pages at logical places (but not within a function). It does not matter
just how long the pages are, since they do not have to fit on a printed
page. The formfeeds should appear alone on lines by themselves.

5.2 Commenting Your Work

Every program should start with a comment saying briefly what it is for.
Example: ‘fmt - filter for simple filling of text’. This comment
should be at the top of the source file containing the ‘main’
function of the program.

Also, please write a brief comment at the start of each source file,
with the file name and a line or two about the overall purpose of the
file.

Please write the comments in a GNU program in English, because English
is the one language that nearly all programmers in all countries can
read. If you do not write English well, please write comments in
English as well as you can, then ask other people to help rewrite them.
If you can’t write comments in English, please find someone to work with
you and translate your comments into English.

Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for. It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion. If there is anything nonstandard about
its use (such as an argument of type char * which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.

Also explain the significance of the return value, if there is one.

Please put two spaces after the end of a sentence in your comments, so
that the Emacs sentence commands will work. Also, please write
complete sentences and capitalize the first word. If a lower-case
identifier comes at the beginning of a sentence, don’t capitalize it!
Changing the spelling makes it a different identifier. If you don’t
like starting a sentence with a lower case letter, write the sentence
differently (e.g., “The identifier lower-case is …”).

The comment on a function is much clearer if you use the argument
names to speak about the argument values. The variable name itself
should be lower case, but write it in upper case when you are speaking
about the value rather than the variable itself. Thus, “the inode
number NODE_NUM” rather than “an inode”.

There is usually no purpose in restating the name of the function in
the comment before it, because readers can see that for themselves.
There might be an exception when the comment is so long that the function
itself would be off the bottom of the screen.

Every ‘#endif’ should have a comment, except in the case of short
conditionals (just a few lines) that are not nested. The comment should
state the condition of the conditional that is ending, including
its sense. ‘#else’ should have a comment describing the condition
and sense of the code that follows. For example:

5.3 Clean Use of C Constructs

Please explicitly declare the types of all objects. For example, you
should explicitly declare all arguments to functions, and you should
declare functions to return int rather than omitting the
int.

Some programmers like to use the GCC ‘-Wall’ option, and change the
code whenever it issues a warning. If you want to do this, then do.
Other programmers prefer not to use ‘-Wall’, because it gives
warnings for valid and legitimate code which they do not want to change.
If you want to do this, then do. The compiler should be your servant,
not your master.

Don’t make the program ugly just to placate static analysis tools such
as lint, clang, and GCC with extra warnings
options such as -Wconversion and -Wundef. These
tools can help find bugs and unclear code, but they can also generate
so many false alarms that it hurts readability to silence them with
unnecessary casts, wrappers, and other complications. For example,
please don’t insert casts to void or calls to do-nothing
functions merely to pacify a lint checker.

Declarations of external functions and functions to appear later in the
source file should all go in one place near the beginning of the file
(somewhere before the first function definition in the file), or else
should go in a header file. Don’t put extern declarations inside
functions.

It used to be common practice to use the same local variables (with
names like tem) over and over for different values within one
function. Instead of doing this, it is better to declare a separate local
variable for each distinct purpose, and give it a name which is
meaningful. This not only makes programs easier to understand, it also
facilitates optimization by good compilers. You can also move the
declaration of each local variable into the smallest scope that includes
all its uses. This makes the program even cleaner.

Don’t use local variables or parameters that shadow global identifiers.
GCC’s ‘-Wshadow’ option can detect this problem.

Don’t declare multiple variables in one declaration that spans lines.
Start a new declaration on each line, instead. For example, instead
of this:

int foo,
bar;

write either this:

int foo, bar;

or this:

int foo;
int bar;

(If they are global variables, each should have a comment preceding it
anyway.)

When you have an if-else statement nested in another
if statement, always put braces around the if-else.
Thus, never write like this:

if (foo)
if (bar)
win ();
else
lose ();

always like this:

if (foo)
{
if (bar)
win ();
else
lose ();
}

If you have an if statement nested inside of an else
statement, either write else if on one line, like this,

if (foo)
…
else if (bar)
…

with its then-part indented like the preceding then-part,
or write the nested if within braces like this:

if (foo)
…
else
{
if (bar)
…
}

Don’t declare both a structure tag and variables or typedefs in the
same declaration. Instead, declare the structure tag separately
and then use it to declare the variables or typedefs.

5.4 Naming Variables, Functions, and Files

The names of global variables and functions in a program serve as
comments of a sort. So don’t choose terse names—instead, look for
names that give useful information about the meaning of the variable or
function. In a GNU program, names should be English, like other
comments.

Local variable names can be shorter, because they are used only within
one context, where (presumably) comments explain their purpose.

Try to limit your use of abbreviations in symbol names. It is ok to
make a few abbreviations, explain what they mean, and then use them
frequently, but don’t use lots of obscure abbreviations.

Please use underscores to separate words in a name, so that the Emacs
word commands can be useful within them. Stick to lower case; reserve
upper case for macros and enum constants, and for name-prefixes
that follow a uniform convention.

For example, you should use names like ignore_space_change_flag;
don’t use names like iCantReadThis.

Variables that indicate whether command-line options have been
specified should be named after the meaning of the option, not after
the option-letter. A comment should state both the exact meaning of
the option and its letter. For example,

When you want to define names with constant integer values, use
enum rather than ‘#define’. GDB knows about enumeration
constants.

You might want to make sure that none of the file names would conflict
if the files were loaded onto an MS-DOS file system which shortens the
names. You can use the program doschk to test for this.

Some GNU programs were designed to limit themselves to file names of 14
characters or less, to avoid file name conflicts if they are read into
older System V systems. Please preserve this feature in the existing
GNU programs that have it, but there is no need to do this in new GNU
programs. doschk also reports file names longer than 14
characters.

5.5 Portability between System Types

In the Unix world, “portability” refers to porting to different Unix
versions. For a GNU program, this kind of portability is desirable, but
not paramount.

The primary purpose of GNU software is to run on top of the GNU kernel,
compiled with the GNU C compiler, on various types of CPU. So the
kinds of portability that are absolutely necessary are quite limited.
But it is important to support Linux-based GNU systems, since they
are the form of GNU that is popular.

Beyond that, it is good to support the other free operating systems
(*BSD), and it is nice to support other Unix-like systems if you want
to. Supporting a variety of Unix-like systems is desirable, although
not paramount. It is usually not too hard, so you may as well do it.
But you don’t have to consider it an obligation, if it does turn out to
be hard.

The easiest way to achieve portability to most Unix-like systems is to
use Autoconf. It’s unlikely that your program needs to know more
information about the host platform than Autoconf can provide, simply
because most of the programs that need such knowledge have already been
written.

Avoid using the format of semi-internal data bases (e.g., directories)
when there is a higher-level alternative (readdir).

As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
and older Macintosh systems, supporting them is often a lot of work.
When that is the case, it is better to spend your time adding features
that will be useful on GNU and GNU/Linux, rather than on supporting
other incompatible systems.

If you do support Windows, please do not abbreviate it as “win”. In
hacker terminology, calling something a “win” is a form of praise.
You’re free to praise Microsoft Windows on your own if you want, but
please don’t do this in GNU packages. Instead of abbreviating
“Windows” to “win”, you can write it in full or abbreviate it to
“woe” or “w”. In GNU Emacs, for instance, we use ‘w32’ in
file names of Windows-specific files, but the macro for Windows
conditionals is called WINDOWSNT.

It is a good idea to define the “feature test macro”
_GNU_SOURCE when compiling your C files. When you compile on GNU
or GNU/Linux, this will enable the declarations of GNU library extension
functions, and that will usually give you a compiler error message if
you define the same function names in some other way in your program.
(You don’t have to actually use these functions, if you prefer
to make the program more portable to other systems.)

But whether or not you use these GNU extensions, you should avoid
using their names for any other meanings. Doing so would make it hard
to move your code into other GNU programs.

5.6 Portability between CPUs

Even GNU systems will differ because of differences among CPU
types—for example, difference in byte ordering and alignment
requirements. It is absolutely essential to handle these differences.
However, don’t make any effort to cater to the possibility that an
int will be less than 32 bits. We don’t support 16-bit machines
in GNU.

Similarly, don’t make any effort to cater to the possibility that
long will be smaller than predefined types like size_t.
For example, the following code is ok:

1989 Standard C requires this to work, and we know of only one
counterexample: 64-bit programs on Microsoft Windows. We will leave
it to those who want to port GNU programs to that environment to
figure out how to do it.

Predefined file-size types like off_t are an exception: they are
longer than long on many platforms, so code like the above won’t
work with them. One way to print an off_t value portably is to
print its digits yourself, one by one.

Don’t assume that the address of an int object is also the
address of its least-significant byte. This is false on big-endian
machines. Thus, don’t make the following mistake:

Avoid casting pointers to integers if you can. Such casts greatly
reduce portability, and in most programs they are easy to avoid. In the
cases where casting pointers to integers is essential—such as, a Lisp
interpreter which stores type information as well as an address in one
word—you’ll have to make explicit provisions to handle different word
sizes. You will also need to make provision for systems in which the
normal range of addresses you can get from malloc starts far away
from zero.

5.7 Calling System Functions

Historically, C implementations differed substantially, and many
systems lacked a full implementation of ANSI/ISO C89. Nowadays,
however, all practical systems have a C89 compiler and GNU C supports
almost all of C99 and some of C11. Similarly, most systems implement
POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.

Hence, there is little reason to support old C or non-POSIX systems,
and you may want to take advantage of standard C and POSIX to write
clearer, more portable, or faster code. You should use standard
interfaces where possible; but if GNU extensions make your program
more maintainable, powerful, or otherwise better, don’t hesitate to
use them. In any case, don’t make your own declaration of system
functions; that’s a recipe for conflict.

Despite the standards, nearly every library function has some sort of
portability issue on some system or another. Here are some examples:

open

Names with trailing /’s are mishandled on many platforms.

printf

long double may be unimplemented; floating values Infinity and
NaN are often mishandled; output for large precisions may be
incorrect.

readlink

May return int instead of ssize_t.

scanf

On Windows, errno is not set on failure.

Gnulib is a big help in
this regard. Gnulib provides implementations of standard interfaces
on many of the systems that lack them, including portable
implementations of enhanced GNU interfaces, thereby making their use
portable, and of POSIX-1.2008 interfaces, some of which are missing
even on up-to-date GNU systems.

Gnulib integrates with GNU Autoconf and Automake to remove much of the
burden of writing portable code from the programmer: Gnulib makes your
configure script automatically determine what features are missing and
use the Gnulib code to supply the missing pieces.

The Gnulib and Autoconf manuals have extensive sections on
portability: Introduction in Gnulib and
see Portable C and C++ in Autoconf. Please consult them
for many more details.

5.8 Internationalization

GNU has a library called GNU gettext that makes it easy to translate the
messages in a program into various languages. You should use this
library in every program. Use English for the messages as they appear
in the program, and let gettext provide the way to translate them into
other languages.

Using GNU gettext involves putting a call to the gettext macro
around each string that might need translation—like this:

printf (gettext ("Processing file '%s'..."), file);

This permits GNU gettext to replace the string "Processing file
'%s'..." with a translated version.

Once a program uses gettext, please make a point of writing calls to
gettext when you add new strings that call for translation.

Using GNU gettext in a package involves specifying a text domain
name for the package. The text domain name is used to separate the
translations for this package from the translations for other packages.
Normally, the text domain name should be the same as the name of the
package—for example, ‘coreutils’ for the GNU core utilities.

To enable gettext to work well, avoid writing code that makes
assumptions about the structure of words or sentences. When you want
the precise text of a sentence to vary depending on the data, use two or
more alternative string constants each containing a complete sentences,
rather than inserting conditionalized words or phrases into a single
sentence framework.

the translator will hardly know that "disk" and "floppy disk" are meant to
be substituted in the other string. Worse, in some languages (like French)
the construction will not work: the translation of the word "full" depends
on the gender of the first part of the sentence; it happens to be not the
same for "disk" as for "floppy disk".

Adding gettext calls to this code cannot give correct results for
all languages, because negation in some languages requires adding words
at more than one place in the sentence. By contrast, adding
gettext calls does the job straightforwardly if the code starts
out like this:

But this still doesn’t work for languages like Polish, which has three
plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
and one for the rest. The GNU ngettext function solves this problem:

5.9 Character Set

Sticking to the ASCII character set (plain text, 7-bit characters) is
preferred in GNU source code comments, text documents, and other
contexts, unless there is good reason to do something else because of
the application domain. For example, if source code deals with the
French Revolutionary calendar, it is OK if its literal strings contain
accented characters in month names like “Floréal”. Also, it is OK
(but not required) to use non-ASCII characters to represent proper
names of contributors in change logs (see Change Logs).

If you need to use non-ASCII characters, you should normally stick
with one encoding, certainly within a single file. UTF-8 is likely to
be the best choice.

5.10 Quote Characters

In the C locale, the output of GNU programs should stick to plain
ASCII for quotation characters in messages to users: preferably 0x22
(‘"’) or 0x27 (‘'’) for both opening and closing quotes.
Although GNU programs traditionally used 0x60 (‘`’) for opening
and 0x27 (‘'’) for closing quotes, nowadays quotes ‘`like
this'’ are typically rendered asymmetrically, so quoting ‘"like
this"’ or ‘'like this'’ typically looks better.

It is ok, but not required, for GNU programs to generate
locale-specific quotes in non-C locales. For example:

printf (gettext ("Processing file '%s'..."), file);

Here, a French translation might cause gettext to return the
string "Traitement de fichier
‹ %s ›...", yielding quotes
more appropriate for a French locale.

Sometimes a program may need to use opening and closing quotes
directly. By convention, gettext translates the string
‘"`"’ to the opening quote and the string ‘"'"’ to the
closing quote, and a program can use these translations. Generally,
though, it is better to translate quote characters in the context of
longer strings.

If the output of your program is ever likely to be parsed by another
program, it is good to provide an option that makes this parsing
reliable. For example, you could escape special characters using
conventions from the C language or the Bourne shell. See for example
the option --quoting-style of GNU ls.

5.11 Mmap

If you use mmap to read or write files, don’t assume it either
works on all files or fails for all files. It may work on some files
and fail on others.

The proper way to use mmap is to try it on the specific file for
which you want to use it—and if mmap doesn’t work, fall back on
doing the job in another way using read and write.

The reason this precaution is needed is that the GNU kernel (the HURD)
provides a user-extensible file system, in which there can be many
different kinds of “ordinary files”. Many of them support
mmap, but some do not. It is important to make programs handle
all these kinds of files.

6 Documenting Programs

A GNU program should ideally come with full free documentation, adequate
for both reference and tutorial purposes. If the package can be
programmed or extended, the documentation should cover programming or
extending it, as well as just using it.

6.1 GNU Manuals

The preferred document format for the GNU system is the Texinfo
formatting language. Every GNU package should (ideally) have
documentation in Texinfo both for reference and for learners. Texinfo
makes it possible to produce a good quality formatted book, using
TeX, and to generate an Info file. It is also possible to generate
HTML output from Texinfo source. See the Texinfo manual, either the
hardcopy, or the on-line version available through info or the
Emacs Info subsystem (C-h i).

Nowadays some other formats such as Docbook and Sgmltexi can be
converted automatically into Texinfo. It is ok to produce the Texinfo
documentation by conversion this way, as long as it gives good results.

Make sure your manual is clear to a reader who knows nothing about the
topic and reads it straight through. This means covering basic topics
at the beginning, and advanced topics only later. This also means
defining every specialized term when it is first used.

Programmers tend to carry over the structure of the program as the
structure for its documentation. But this structure is not
necessarily good for explaining how to use the program; it may be
irrelevant and confusing for a user.

Instead, the right way to structure documentation is according to the
concepts and questions that a user will have in mind when reading it.
This principle applies at every level, from the lowest (ordering
sentences in a paragraph) to the highest (ordering of chapter topics
within the manual). Sometimes this structure of ideas matches the
structure of the implementation of the software being documented—but
often they are different. An important part of learning to write good
documentation is to learn to notice when you have unthinkingly
structured the documentation like the implementation, stop yourself,
and look for better alternatives.

For example, each program in the GNU system probably ought to be
documented in one manual; but this does not mean each program should
have its own manual. That would be following the structure of the
implementation, rather than the structure that helps the user
understand.

Instead, each manual should cover a coherent topic. For example,
instead of a manual for diff and a manual for diff3, we
have one manual for “comparison of files” which covers both of those
programs, as well as cmp. By documenting these programs
together, we can make the whole subject clearer.

The manual which discusses a program should certainly document all of
the program’s command-line options and all of its commands. It should
give examples of their use. But don’t organize the manual as a list
of features. Instead, organize it logically, by subtopics. Address
the questions that a user will ask when thinking about the job that
the program does. Don’t just tell the reader what each feature can
do—say what jobs it is good for, and show how to use it for those
jobs. Explain what is recommended usage, and what kinds of usage
users should avoid.

In general, a GNU manual should serve both as tutorial and reference.
It should be set up for convenient access to each topic through Info,
and for reading straight through (appendixes aside). A GNU manual
should give a good introduction to a beginner reading through from the
start, and should also provide all the details that hackers want.
The Bison manual is a good example of this—please take a look at it
to see what we mean.

That is not as hard as it first sounds. Arrange each chapter as a
logical breakdown of its topic, but order the sections, and write their
text, so that reading the chapter straight through makes sense. Do
likewise when structuring the book into chapters, and when structuring a
section into paragraphs. The watchword is, at each point, address
the most fundamental and important issue raised by the preceding text.

If necessary, add extra chapters at the beginning of the manual which
are purely tutorial and cover the basics of the subject. These provide
the framework for a beginner to understand the rest of the manual. The
Bison manual provides a good example of how to do this.

To serve as a reference, a manual should have an Index that lists all
the functions, variables, options, and important concepts that are
part of the program. One combined Index should do for a short manual,
but sometimes for a complex package it is better to use multiple
indices. The Texinfo manual includes advice on preparing good index
entries, see Making Index Entries in GNU
Texinfo, and see Defining the Entries of an
Index in GNU Texinfo.

Don’t use Unix man pages as a model for how to write GNU documentation;
most of them are terse, badly structured, and give inadequate
explanation of the underlying concepts. (There are, of course, some
exceptions.) Also, Unix man pages use a particular format which is
different from what we use in GNU manuals.

Please include an email address in the manual for where to report
bugs in the text of the manual.

Please do not use the term “pathname” that is used in Unix
documentation; use “file name” (two words) instead. We use the term
“path” only for search paths, which are lists of directory names.

Please do not use the term “illegal” to refer to erroneous input to
a computer program. Please use “invalid” for this, and reserve the
term “illegal” for activities prohibited by law.

Please do not write ‘()’ after a function name just to indicate
it is a function. foo () is not a function, it is a function
call with no arguments.

6.2 Doc Strings and Manuals

Some programming systems, such as Emacs, provide a documentation string
for each function, command or variable. You may be tempted to write a
reference manual by compiling the documentation strings and writing a
little additional text to go around them—but you must not do it. That
approach is a fundamental mistake. The text of well-written
documentation strings will be entirely wrong for a manual.

A documentation string needs to stand alone—when it appears on the
screen, there will be no other text to introduce or explain it.
Meanwhile, it can be rather informal in style.

The text describing a function or variable in a manual must not stand
alone; it appears in the context of a section or subsection. Other text
at the beginning of the section should explain some of the concepts, and
should often make some general points that apply to several functions or
variables. The previous descriptions of functions and variables in the
section will also have given information about the topic. A description
written to stand alone would repeat some of that information; this
redundancy looks bad. Meanwhile, the informality that is acceptable in
a documentation string is totally unacceptable in a manual.

The only good way to use documentation strings in writing a good manual
is to use them as a source of information for writing good text.

6.3 Manual Structure Details

The title page of the manual should state the version of the programs or
packages documented in the manual. The Top node of the manual should
also contain this information. If the manual is changing more
frequently than or independent of the program, also state a version
number for the manual in both of these places.

Each program documented in the manual should have a node named
‘program Invocation’ or ‘Invoking program’. This
node (together with its subnodes, if any) should describe the program’s
command line arguments and how to run it (the sort of information people
would look for in a man page). Start with an ‘@example’
containing a template for all the options and arguments that the program
uses.

Alternatively, put a menu item in some menu whose item name fits one of
the above patterns. This identifies the node which that item points to
as the node for this purpose, regardless of the node’s actual name.

The ‘--usage’ feature of the Info reader looks for such a node
or menu item in order to find the relevant text, so it is essential
for every Texinfo file to have one.

If one manual describes several programs, it should have such a node for
each program described in the manual.

6.4 License for Manuals

Please use the GNU Free Documentation License for all GNU manuals that
are more than a few pages long. Likewise for a collection of short
documents—you only need one copy of the GNU FDL for the whole
collection. For a single short document, you can use a very permissive
non-copyleft license, to avoid taking up space with a long license.

Note that it is not obligatory to include a copy of the GNU GPL or GNU
LGPL in a manual whose license is neither the GPL nor the LGPL. It can
be a good idea to include the program’s license in a large manual; in a
short manual, whose size would be increased considerably by including
the program’s license, it is probably better not to include it.

6.5 Manual Credits

Please credit the principal human writers of the manual as the authors,
on the title page of the manual. If a company sponsored the work, thank
the company in a suitable place in the manual, but do not cite the
company as an author.

6.6 Printed Manuals

The FSF publishes some GNU manuals in printed form. To encourage sales
of these manuals, the on-line versions of the manual should mention at
the very start that the printed manual is available and should point at
information for getting it—for instance, with a link to the page
http://www.gnu.org/order/order.html. This should not be included
in the printed manual, though, because there it is redundant.

It is also useful to explain in the on-line forms of the manual how the
user can print out the manual from the sources.

6.7 The NEWS File

In addition to its manual, the package should have a file named
NEWS which contains a list of user-visible changes worth
mentioning. In each new release, add items to the front of the file and
identify the version they pertain to. Don’t discard old items; leave
them in the file after the newer items. This way, a user upgrading from
any previous version can see what is new.

If the NEWS file gets very long, move some of the older items
into a file named ONEWS and put a note at the end referring the
user to that file.

6.8 Change Logs

Keep a change log to describe all the changes made to program source
files. The purpose of this is so that people investigating bugs in the
future will know about the changes that might have introduced the bug.
Often a new bug can be found by looking at what was recently changed.
More importantly, change logs can help you eliminate conceptual
inconsistencies between different parts of a program, by giving you a
history of how the conflicting concepts arose and who they came from.

6.8.1 Change Log Concepts

You can think of the change log as a conceptual “undo list” which
explains how earlier versions were different from the current version.
People can see the current version; they don’t need the change log to
tell them what is in it. What they want from a change log is a clear
explanation of how the earlier version differed. Each entry in
a change log describes either an individual change or the smallest
batch of changes that belong together, also known as a change
set.
For later reference or for summarizing, sometimes it is useful to
start the entry with a one-line description (sometimes called a
title) to describe its overall purpose.

In the past, we recommended not mentioning changes in non-software
files (manuals, help files, media files, etc.) in change logs.
However, we’ve been advised that it is a good idea to include them,
for the sake of copyright records.

The change log file is normally called ChangeLog and covers an
entire directory. Each directory can have its own change log, or a
directory can use the change log of its parent directory—it’s up to
you.

Another alternative is to record change log information with a version
control system such as RCS or CVS. This can be converted automatically
to a ChangeLog file using rcs2log; in Emacs, the command
C-x v a (vc-update-change-log) does the job.

For changes to code, there’s no need to describe the full purpose of
the changes or how they work together. If you think that a change
calls for explanation, you’re probably right. Please do explain
it—but please put the full explanation in comments in the code,
where people will see it whenever they see the code. For example,
“New function” is enough for the change log when you add a function,
because there should be a comment before the function definition to
explain what it does.

For changes to files that do not support a comment syntax (e.g., media
files), it is ok to include the full explanation in the change log file,
after the title and before the list of individual changes.

The easiest way to add an entry to ChangeLog is with the Emacs
command M-x add-change-log-entry. An individual change should
have an asterisk, the name of the changed file, and then in
parentheses the name of the changed functions, variables or whatever,
followed by a colon. Then describe the changes you made to that
function or variable.

6.8.2 Style of Change Logs

Here are some simple examples of change log entries, starting with the
header line that says who made the change and when it was installed,
followed by descriptions of specific changes. (These examples are
drawn from Emacs and GCC.)

It’s important to name the changed function or variable in full. Don’t
abbreviate function or variable names, and don’t combine them.
Subsequent maintainers will often search for a function name to find all
the change log entries that pertain to it; if you abbreviate the name,
they won’t find it when they search.

For example, some people are tempted to abbreviate groups of function
names by writing ‘* register.el ({insert,jump-to}-register)’;
this is not a good idea, since searching for jump-to-register or
insert-register would not find that entry.

Separate unrelated change log entries with blank lines. Don’t put
blank lines between individual changes of an entry. You can omit the
file name and the asterisk when successive individual changes are in
the same file.

Break long lists of function names by closing continued lines with
‘)’, rather than ‘,’, and opening the continuation with
‘(’ as in this example:

6.8.3 Simple Changes

Certain simple kinds of changes don’t need much detail in the change
log.

When you change the calling sequence of a function in a simple fashion,
and you change all the callers of the function to use the new calling
sequence, there is no need to make individual entries for all the
callers that you changed. Just write in the entry for the function
being called, “All callers changed”—like this:

When you change just comments or doc strings, it is enough to write an
entry for the file, without mentioning the functions. Just “Doc
fixes” is enough for the change log.

There’s no technical need to make change log entries for documentation
files. This is because documentation is not susceptible to bugs that
are hard to fix. Documentation does not consist of parts that must
interact in a precisely engineered fashion. To correct an error, you
need not know the history of the erroneous passage; it is enough to
compare what the documentation says with the way the program actually
works.

However, you should keep change logs for documentation files when the
project gets copyright assignments from its contributors, so as to
make the records of authorship more accurate.

6.8.4 Conditional Changes

Source files can often contain code that is conditional to build-time
or static conditions. For example, C programs can contain
compile-time #if conditionals; programs implemented in
interpreted languages can contain module imports of function
definitions that are only performed for certain versions of the
interpreter; and Automake Makefile.am files can contain
variable definitions or target declarations that are only to be
considered if a configure-time Automake conditional is true.

Many changes are conditional as well: sometimes you add a new variable,
or function, or even a new program or library, which is entirely
dependent on a build-time condition. It is useful to indicate
in the change log the conditions for which a change applies.

Our convention for indicating conditional changes is to use
square brackets around the name of the condition.

Conditional changes can happen in numerous scenarios and with many
variations, so here are some examples to help clarify. This first
example describes changes in C, Perl, and Python files which are
conditional but do not have an associated function or entity name:

6.8.5 Indicating the Part Changed

Indicate the part of a function which changed by using angle brackets
enclosing an indication of what the changed part does. Here is an entry
for a change in the part of the function sh-while-getopts that
deals with sh commands:

6.9 Man Pages

In the GNU project, man pages are secondary. It is not necessary or
expected for every GNU program to have a man page, but some of them do.
It’s your choice whether to include a man page in your program.

When you make this decision, consider that supporting a man page
requires continual effort each time the program is changed. The time
you spend on the man page is time taken away from more useful work.

For a simple program which changes little, updating the man page may be
a small job. Then there is little reason not to include a man page, if
you have one.

For a large program that changes a great deal, updating a man page may
be a substantial burden. If a user offers to donate a man page, you may
find this gift costly to accept. It may be better to refuse the man
page unless the same person agrees to take full responsibility for
maintaining it—so that you can wash your hands of it entirely. If
this volunteer later ceases to do the job, then don’t feel obliged to
pick it up yourself; it may be better to withdraw the man page from the
distribution until someone else agrees to update it.

When a program changes only a little, you may feel that the
discrepancies are small enough that the man page remains useful without
updating. If so, put a prominent note near the beginning of the man
page explaining that you don’t maintain it and that the Texinfo manual
is more authoritative. The note should say how to access the Texinfo
documentation.

Be sure that man pages include a copyright statement and free license.
The simple all-permissive license is appropriate for simple man pages
(see License Notices for Other Files in Information for GNU
Maintainers).

For long man pages, with enough explanation and documentation that
they can be considered true manuals, use the GFDL (see License for Manuals).

Finally, the GNU help2man program
(http://www.gnu.org/software/help2man/) is one way to automate
generation of a man page, in this case from --help output.
This is sufficient in many cases.

6.10 Reading other Manuals

There may be non-free books or documentation files that describe the
program you are documenting.

It is ok to use these documents for reference, just as the author of a
new algebra textbook can read other books on algebra. A large portion
of any non-fiction book consists of facts, in this case facts about how
a certain program works, and these facts are necessarily the same for
everyone who writes about the subject. But be careful not to copy your
outline structure, wording, tables or examples from preexisting non-free
documentation. Copying from free documentation may be ok; please check
with the FSF about the individual case.

7 The Release Process

Making a release is more than just bundling up your source files in a
tar file and putting it up for FTP. You should set up your software so
that it can be configured to run on a variety of systems. Your Makefile
should conform to the GNU standards described below, and your directory
layout should also conform to the standards discussed below. Doing so
makes it easy to include your package into the larger framework of
all GNU software.

7.1 How Configuration Should Work

Each GNU distribution should come with a shell script named
configure. This script is given arguments which describe the
kind of machine and system you want to compile the program for.
The configure script must record the configuration options so
that they affect compilation.

The description here is the specification of the interface for the
configure script in GNU packages. Many packages implement it
using GNU Autoconf (see Introduction in Autoconf)
and/or GNU Automake (see Introduction in Automake),
but you do not have to use these tools. You can implement it any way
you like; for instance, by making configure be a wrapper around
a completely different configuration system.

Another way for the configure script to operate is to make a
link from a standard name such as config.h to the proper
configuration file for the chosen system. If you use this technique,
the distribution should not contain a file named
config.h. This is so that people won’t be able to build the
program without configuring it first.

Another thing that configure can do is to edit the Makefile. If
you do this, the distribution should not contain a file named
Makefile. Instead, it should include a file Makefile.in which
contains the input used for editing. Once again, this is so that people
won’t be able to build the program without configuring it first.

If configure does write the Makefile, then Makefile
should have a target named Makefile which causes configure
to be rerun, setting up the same configuration that was set up last
time. The files that configure reads should be listed as
dependencies of Makefile.

All the files which are output from the configure script should
have comments at the beginning explaining that they were generated
automatically using configure. This is so that users won’t think
of trying to edit them by hand.

The configure script should write a file named config.status
which describes which configuration options were specified when the
program was last configured. This file should be a shell script which,
if run, will recreate the same configuration.

The configure script should accept an option of the form
‘--srcdir=dirname’ to specify the directory where sources are found
(if it is not the current directory). This makes it possible to build
the program in a separate directory, so that the actual source directory
is not modified.

If the user does not specify ‘--srcdir’, then configure should
check both . and .. to see if it can find the sources. If
it finds the sources in one of these places, it should use them from
there. Otherwise, it should report that it cannot find the sources, and
should exit with nonzero status.

Usually the easy way to support ‘--srcdir’ is by editing a
definition of VPATH into the Makefile. Some rules may need to
refer explicitly to the specified source directory. To make this
possible, configure can add to the Makefile a variable named
srcdir whose value is precisely the specified directory.

In addition, the ‘configure’ script should take options
corresponding to most of the standard directory variables
(see Directory Variables). Here is the list:

The configure script should also take an argument which specifies the
type of system to build the program for. This argument should look like
this:

cpu-company-system

For example, an Athlon-based GNU/Linux system might be
‘i686-pc-linux-gnu’.

The configure script needs to be able to decode all plausible
alternatives for how to describe a machine. Thus,
‘athlon-pc-gnu/linux’ would be a valid alias. There is a shell
script called
config.sub that you can use as a subroutine to validate system
types and canonicalize aliases.

The configure script should also take the option
--build=buildtype, which should be equivalent to a
plain buildtype argument. For example, ‘configure
--build=i686-pc-linux-gnu’ is equivalent to ‘configure
i686-pc-linux-gnu’. When the build type is not specified by an option
or argument, the configure script should normally guess it using
the shell script
config.guess.

Other options are permitted to specify in more detail the software
or hardware present on the machine, to include or exclude optional parts
of the package, or to adjust the name of some tools or arguments to them:

‘--enable-feature[=parameter]’

Configure the package to build and install an optional user-level
facility called feature. This allows users to choose which
optional features to include. Giving an optional parameter of
‘no’ should omit feature, if it is built by default.

No ‘--enable’ option should ever cause one feature to
replace another. No ‘--enable’ option should ever substitute one
useful behavior for another useful behavior. The only proper use for
‘--enable’ is for questions of whether to build part of the program
or exclude it.

‘--with-package’

The package package will be installed, so configure this package
to work with package.

Do not use a ‘--with’ option to specify the file name to use to
find certain files. That is outside the scope of what ‘--with’
options are for.

‘variable=value’

Set the value of the variable variable to value. This is
used to override the default values of commands or arguments in the
build process. For example, the user could issue ‘configure
CFLAGS=-g CXXFLAGS=-g’ to build with debugging information and without
the default optimization.

Specifying variables as arguments to configure, like this:

./configure CC=gcc

is preferable to setting them in environment variables:

CC=gcc ./configure

as it helps to recreate the same configuration later with
config.status. However, both methods should be supported.

All configure scripts should accept all of the “detail”
options and the variable settings, whether or not they make any
difference to the particular package at hand. In particular, they
should accept any option that starts with ‘--with-’ or
‘--enable-’. This is so users will be able to configure an
entire GNU source tree at once with a single set of options.

You will note that the categories ‘--with-’ and ‘--enable-’
are narrow: they do not provide a place for any sort of option
you might think of. That is deliberate. We want to limit the possible
configuration options in GNU software. We do not want GNU programs to
have idiosyncratic configuration options.

Packages that perform part of the compilation process may support
cross-compilation. In such a case, the host and target machines for the
program may be different.

The configure script should normally treat the specified type of
system as both the host and the target, thus producing a program which
works for the same type of machine that it runs on.

To compile a program to run on a host type that differs from the build
type, use the configure option --host=hosttype, where
hosttype uses the same syntax as buildtype. The host type
normally defaults to the build type.

To configure a cross-compiler, cross-assembler, or what have you, you
should specify a target different from the host, using the configure
option ‘--target=targettype’. The syntax for
targettype is the same as for the host type. So the command would
look like this:

./configure --host=hosttype --target=targettype

The target type normally defaults to the host type.
Programs for which cross-operation is not meaningful need not accept the
‘--target’ option, because configuring an entire operating system for
cross-operation is not a meaningful operation.

Some programs have ways of configuring themselves automatically. If
your program is set up to do this, your configure script can simply
ignore most of its arguments.

7.2 Makefile Conventions

This
describes conventions for writing the Makefiles for GNU programs.
Using Automake will help you write a Makefile that follows these
conventions. For more information on portable Makefiles, see
POSIX and Portable Make Programming in Autoconf.

7.2.1 General Conventions for Makefiles

Every Makefile should contain this line:

SHELL = /bin/sh

to avoid trouble on systems where the SHELL variable might be
inherited from the environment. (This is never a problem with GNU
make.)

Different make programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:

.SUFFIXES:
.SUFFIXES: .c .o

The first line clears out the suffix list, the second introduces all
suffixes which may be subject to implicit rules in this Makefile.

Don’t assume that . is in the path for command execution. When
you need to run programs that are a part of your package during the
make, please make sure that it uses ./ if the program is built as
part of the make or $(srcdir)/ if the file is an unchanging part
of the source code. Without one of these prefixes, the current search
path is used.

The distinction between ./ (the build directory) and
$(srcdir)/ (the source directory) is important because
users can build in a separate directory using the ‘--srcdir’ option
to configure. A rule of the form:

foo.1 : foo.man sedscript
sed -f sedscript foo.man > foo.1

will fail when the build directory is not the source directory, because
foo.man and sedscript are in the source directory.

When using GNU make, relying on ‘VPATH’ to find the source
file will work in the case where there is a single dependency file,
since the make automatic variable ‘$<’ will represent the
source file wherever it is. (Many versions of make set ‘$<’
only in implicit rules.) A Makefile target like

foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o

should instead be written as

foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@

in order to allow ‘VPATH’ to work correctly. When the target has
multiple dependencies, using an explicit ‘$(srcdir)’ is the easiest
way to make the rule work well. For example, the target above for
foo.1 is best written as:

GNU distributions usually contain some files which are not source
files—for example, Info files, and the output from Autoconf, Automake,
Bison or Flex. Since these files normally appear in the source
directory, they should always appear in the source directory, not in the
build directory. So Makefile rules to update them should put the
updated files in the source directory.

However, if a file does not appear in the distribution, then the
Makefile should not put it in the source directory, because building a
program in ordinary circumstances should not modify the source directory
in any way.

Try to make the build and installation targets, at least (and all their
subtargets) work correctly with a parallel make.

7.2.2 Utilities in Makefiles

Write the Makefile commands (and any shell scripts, such as
configure) to run under sh (both the traditional Bourne
shell and the POSIX shell), not csh. Don’t use any
special features of ksh or bash, or POSIX features
not widely supported in traditional Bourne sh.

The configure script and the Makefile rules for building and
installation should not use any utilities directly except these:

Generally, stick to the widely-supported (usually
POSIX-specified) options and features of these programs. For
example, don’t use ‘mkdir -p’, convenient as it may be, because a
few systems don’t support it at all and with others, it is not safe
for parallel execution. For a list of known incompatibilities, see
Portable Shell Programming in Autoconf.

It is a good idea to avoid creating symbolic links in makefiles, since a
few file systems don’t support them.

The Makefile rules for building and installation can also use compilers
and related programs, but should do so via make variables so that the
user can substitute alternatives. Here are some of the programs we
mean:

When you use ranlib or ldconfig, you should make sure
nothing bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf ‘AC_PROG_RANLIB’ macro can help with
this.)

If you use symbolic links, you should implement a fallback for systems
that don’t have symbolic links.

Additional utilities that can be used via Make variables are:

chgrp chmod chown mknod

It is ok to use other utilities in Makefile portions (or scripts)
intended only for particular systems where you know those utilities
exist.

7.2.3 Variables for Specifying Commands

Makefiles should provide variables for overriding certain commands, options,
and so on.

In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named BISON whose default
value is set with ‘BISON = bison’, and refer to it with
$(BISON) whenever you need to use Bison.

File management utilities such as ln, rm, mv, and
so on, need not be referred to through variables in this way, since users
don’t need to replace them with other programs.

Each program-name variable should come with an options variable that is
used to supply options to the program. Append ‘FLAGS’ to the
program-name variable name to get the options variable name—for
example, BISONFLAGS. (The names CFLAGS for the C
compiler, YFLAGS for yacc, and LFLAGS for lex, are
exceptions to this rule, but we keep them because they are standard.)
Use CPPFLAGS in any compilation command that runs the
preprocessor, and use LDFLAGS in any compilation command that
does linking as well as in any direct use of ld.

If there are C compiler options that must be used for proper
compilation of certain files, do not include them in CFLAGS.
Users expect to be able to specify CFLAGS freely themselves.
Instead, arrange to pass the necessary options to the C compiler
independently of CFLAGS, by writing them explicitly in the
compilation commands or by defining an implicit rule, like this:

Do include the ‘-g’ option in CFLAGS, because that is not
required for proper compilation. You can consider it a default
that is only recommended. If the package is set up so that it is
compiled with GCC by default, then you might as well include ‘-O’
in the default value of CFLAGS as well.

Put CFLAGS last in the compilation command, after other variables
containing compiler options, so the user can use CFLAGS to
override the others.

CFLAGS should be used in every invocation of the C compiler,
both those which do compilation and those which do linking.

Every Makefile should define the variable INSTALL, which is the
basic command for installing a file into the system.

Every Makefile should also define the variables INSTALL_PROGRAM
and INSTALL_DATA. (The default for INSTALL_PROGRAM should
be $(INSTALL); the default for INSTALL_DATA should be
${INSTALL} -m 644.) Then it should use those variables as the
commands for actual installation, for executables and non-executables
respectively. Minimal use of these variables is as follows:

7.2.4 DESTDIR: Support for Staged Installs

The DESTDIR variable is specified by the user on the make
command line as an absolute file name. For example:

make DESTDIR=/tmp/stage install

DESTDIR should be supported only in the install* and
uninstall* targets, as those are the only targets where it is
useful.

If your installation step would normally install
/usr/local/bin/foo and /usr/local/lib/libfoo.a, then an
installation invoked as in the example above would install
/tmp/stage/usr/local/bin/foo and
/tmp/stage/usr/local/lib/libfoo.a instead.

Prepending the variable DESTDIR to each target in this way
provides for staged installs, where the installed files are not
placed directly into their expected location but are instead copied
into a temporary location (DESTDIR). However, installed files
maintain their relative directory structure and any embedded file names
will not be modified.

You should not set the value of DESTDIR in your Makefile
at all; then the files are installed into their expected locations by
default. Also, specifying DESTDIR should not change the
operation of the software in any way, so its value should not be
included in any file contents.

DESTDIR support is commonly used in package creation. It is
also helpful to users who want to understand what a given package will
install where, and to allow users who don’t normally have permissions
to install into protected areas to build and install before gaining
those permissions. Finally, it can be useful with tools such as
stow, where code is installed in one place but made to appear
to be installed somewhere else using symbolic links or special mount
operations. So, we strongly recommend GNU packages support
DESTDIR, though it is not an absolute requirement.

7.2.5 Variables for Installation Directories

Installation directories should always be named by variables, so it is
easy to install in a nonstandard place. The standard names for these
variables and the values they should have in GNU packages are
described below. They are based on a standard file system layout;
variants of it are used in GNU/Linux and other modern operating
systems.

Installers are expected to override these values when calling
make (e.g., make prefix=/usr install or
configure (e.g., configure --prefix=/usr). GNU
packages should not try to guess which value should be appropriate for
these variables on the system they are being installed onto: use the
default settings specified here so that all GNU packages behave
identically, allowing the installer to achieve any desired layout.

All installation directories, and their parent directories, should be
created (if necessary) before they are installed into.

These first two variables set the root for the installation. All the
other installation directories should be subdirectories of one of
these two, and nothing should be directly installed into these two
directories.

prefix

A prefix used in constructing the default values of the variables listed
below. The default value of prefix should be /usr/local.
When building the complete GNU system, the prefix will be empty and
/usr will be a symbolic link to /.
(If you are using Autoconf, write it as ‘@prefix@’.)

Running ‘make install’ with a different value of prefix from
the one used to build the program should not recompile the
program.

exec_prefix

A prefix used in constructing the default values of some of the
variables listed below. The default value of exec_prefix should
be $(prefix).
(If you are using Autoconf, write it as ‘@exec_prefix@’.)

Generally, $(exec_prefix) is used for directories that contain
machine-specific files (such as executables and subroutine libraries),
while $(prefix) is used directly for other directories.

Running ‘make install’ with a different value of exec_prefix
from the one used to build the program should not recompile the
program.

Executable programs are installed in one of the following directories.

bindir

The directory for installing executable programs that users can run.
This should normally be /usr/local/bin, but write it as
$(exec_prefix)/bin.
(If you are using Autoconf, write it as ‘@bindir@’.)

sbindir

The directory for installing executable programs that can be run from
the shell, but are only generally useful to system administrators. This
should normally be /usr/local/sbin, but write it as
$(exec_prefix)/sbin.
(If you are using Autoconf, write it as ‘@sbindir@’.)

libexecdir

The directory for installing executable programs to be run by other
programs rather than by users. This directory should normally be
/usr/local/libexec, but write it as $(exec_prefix)/libexec.
(If you are using Autoconf, write it as ‘@libexecdir@’.)

The definition of ‘libexecdir’ is the same for all packages, so
you should install your data in a subdirectory thereof. Most packages
install their data under $(libexecdir)/package-name/,
possibly within additional subdirectories thereof, such as
$(libexecdir)/package-name/machine/version.

Data files used by the program during its execution are divided into
categories in two ways.

Some files are normally modified by programs; others are never normally
modified (though users may edit some of these).

Some files are architecture-independent and can be shared by all
machines at a site; some are architecture-dependent and can be shared
only by machines of the same kind and operating system; others may never
be shared between two machines.

This makes for six different possibilities. However, we want to
discourage the use of architecture-dependent files, aside from object
files and libraries. It is much cleaner to make other data files
architecture-independent, and it is generally not hard.

Here are the variables Makefiles should use to specify directories
to put these various kinds of files in:

‘datarootdir’

The root of the directory tree for read-only architecture-independent
data files. This should normally be /usr/local/share, but
write it as $(prefix)/share. (If you are using Autoconf, write
it as ‘@datarootdir@’.) ‘datadir’’s default value is
based on this variable; so are ‘infodir’, ‘mandir’, and
others.

‘datadir’

The directory for installing idiosyncratic read-only
architecture-independent data files for this program. This is usually
the same place as ‘datarootdir’, but we use the two separate
variables so that you can move these program-specific files without
altering the location for Info files, man pages, etc.

This should normally be /usr/local/share, but write it as
$(datarootdir). (If you are using Autoconf, write it as
‘@datadir@’.)

The definition of ‘datadir’ is the same for all packages, so you
should install your data in a subdirectory thereof. Most packages
install their data under $(datadir)/package-name/.

‘sysconfdir’

The directory for installing read-only data files that pertain to a
single machine–that is to say, files for configuring a host. Mailer
and network configuration files, /etc/passwd, and so forth belong
here. All the files in this directory should be ordinary ASCII text
files. This directory should normally be /usr/local/etc, but
write it as $(prefix)/etc.
(If you are using Autoconf, write it as ‘@sysconfdir@’.)

Do not install executables here in this directory (they probably belong
in $(libexecdir) or $(sbindir)). Also do not install
files that are modified in the normal course of their use (programs
whose purpose is to change the configuration of the system excluded).
Those probably belong in $(localstatedir).

‘sharedstatedir’

The directory for installing architecture-independent data files which
the programs modify while they run. This should normally be
/usr/local/com, but write it as $(prefix)/com.
(If you are using Autoconf, write it as ‘@sharedstatedir@’.)

‘localstatedir’

The directory for installing data files which the programs modify while
they run, and that pertain to one specific machine. Users should never
need to modify files in this directory to configure the package’s
operation; put such configuration information in separate files that go
in $(datadir) or $(sysconfdir). $(localstatedir)
should normally be /usr/local/var, but write it as
$(prefix)/var.
(If you are using Autoconf, write it as ‘@localstatedir@’.)

‘runstatedir’

The directory for installing data files which the programs modify
while they run, that pertain to one specific machine, and which need
not persist longer than the execution of the program—which is
generally long-lived, for example, until the next reboot. PID files
for system daemons are a typical use. In addition, this directory
should not be cleaned except perhaps at reboot, while the general
/tmp (TMPDIR) may be cleaned arbitrarily. This should
normally be /var/run, but write it as
$(localstatedir)/run. Having it as a separate variable allows
the use of /run if desired, for example. (If you are using
Autoconf 2.70 or later, write it as ‘@runstatedir@’.)

These variables specify the directory for installing certain specific
types of files, if your program has them. Every GNU package should
have Info files, so every program needs ‘infodir’, but not all
need ‘libdir’ or ‘lispdir’.

‘includedir’

The directory for installing header files to be included by user
programs with the C ‘#include’ preprocessor directive. This
should normally be /usr/local/include, but write it as
$(prefix)/include.
(If you are using Autoconf, write it as ‘@includedir@’.)

Most compilers other than GCC do not look for header files in directory
/usr/local/include. So installing the header files this way is
only useful with GCC. Sometimes this is not a problem because some
libraries are only really intended to work with GCC. But some libraries
are intended to work with other compilers. They should install their
header files in two places, one specified by includedir and one
specified by oldincludedir.

‘oldincludedir’

The directory for installing ‘#include’ header files for use with
compilers other than GCC. This should normally be /usr/include.
(If you are using Autoconf, you can write it as ‘@oldincludedir@’.)

The Makefile commands should check whether the value of
oldincludedir is empty. If it is, they should not try to use
it; they should cancel the second installation of the header files.

A package should not replace an existing header in this directory unless
the header came from the same package. Thus, if your Foo package
provides a header file foo.h, then it should install the header
file in the oldincludedir directory if either (1) there is no
foo.h there or (2) the foo.h that exists came from the Foo
package.

To tell whether foo.h came from the Foo package, put a magic
string in the file—part of a comment—and grep for that string.

‘docdir’

The directory for installing documentation files (other than Info) for
this package. By default, it should be
/usr/local/share/doc/yourpkg, but it should be written as
$(datarootdir)/doc/yourpkg. (If you are using Autoconf,
write it as ‘@docdir@’.) The yourpkg subdirectory, which
may include a version number, prevents collisions among files with
common names, such as README.

‘infodir’

The directory for installing the Info files for this package. By
default, it should be /usr/local/share/info, but it should be
written as $(datarootdir)/info. (If you are using Autoconf,
write it as ‘@infodir@’.) infodir is separate from
docdir for compatibility with existing practice.

‘htmldir’

‘dvidir’

‘pdfdir’

‘psdir’

Directories for installing documentation files in the particular
format. They should all be set to $(docdir) by default. (If
you are using Autoconf, write them as ‘@htmldir@’,
‘@dvidir@’, etc.) Packages which supply several translations
of their documentation should install them in
‘$(htmldir)/’ll, ‘$(pdfdir)/’ll, etc. where
ll is a locale abbreviation such as ‘en’ or ‘pt_BR’.

‘libdir’

The directory for object files and libraries of object code. Do not
install executables here, they probably ought to go in $(libexecdir)
instead. The value of libdir should normally be
/usr/local/lib, but write it as $(exec_prefix)/lib.
(If you are using Autoconf, write it as ‘@libdir@’.)

‘lispdir’

The directory for installing any Emacs Lisp files in this package. By
default, it should be /usr/local/share/emacs/site-lisp, but it
should be written as $(datarootdir)/emacs/site-lisp.

If you are using Autoconf, write the default as ‘@lispdir@’.
In order to make ‘@lispdir@’ work, you need the following lines
in your configure.ac file:

lispdir='${datarootdir}/emacs/site-lisp'
AC_SUBST(lispdir)

‘localedir’

The directory for installing locale-specific message catalogs for this
package. By default, it should be /usr/local/share/locale, but
it should be written as $(datarootdir)/locale. (If you are
using Autoconf, write it as ‘@localedir@’.) This directory
usually has a subdirectory per locale.

Unix-style man pages are installed in one of the following:

‘mandir’

The top-level directory for installing the man pages (if any) for this
package. It will normally be /usr/local/share/man, but you
should write it as $(datarootdir)/man. (If you are using
Autoconf, write it as ‘@mandir@’.)

‘man1dir’

The directory for installing section 1 man pages. Write it as
$(mandir)/man1.

‘man2dir’

The directory for installing section 2 man pages. Write it as
$(mandir)/man2

‘…’

Don’t make the primary documentation for any GNU software be a
man page. Write a manual in Texinfo instead. Man pages are just for
the sake of people running GNU software on Unix, which is a secondary
application only.

‘manext’

The file name extension for the installed man page. This should contain
a period followed by the appropriate digit; it should normally be ‘.1’.

‘man1ext’

The file name extension for installed section 1 man pages.

‘man2ext’

The file name extension for installed section 2 man pages.

‘…’

Use these names instead of ‘manext’ if the package needs to install man
pages in more than one section of the manual.

And finally, you should set the following variable:

‘srcdir’

The directory for the sources being compiled. The value of this
variable is normally inserted by the configure shell script.
(If you are using Autoconf, use ‘srcdir = @srcdir@’.)

For example:

# Common prefix for installation directories.
# NOTE: This directory must exist when you start the install.
prefix = /usr/local
datarootdir = $(prefix)/share
datadir = $(datarootdir)
exec_prefix = $(prefix)
# Where to put the executable for the command 'gcc'.
bindir = $(exec_prefix)/bin
# Where to put the directories used by the compiler.
libexecdir = $(exec_prefix)/libexec
# Where to put the Info files.
infodir = $(datarootdir)/info

If your program installs a large number of files into one of the
standard user-specified directories, it might be useful to group them
into a subdirectory particular to that program. If you do this, you
should write the install rule to create these subdirectories.

Do not expect the user to include the subdirectory name in the value of
any of the variables listed above. The idea of having a uniform set of
variable names for installation directories is to enable the user to
specify the exact same values for several different GNU packages. In
order for this to be useful, all the packages must be designed so that
they will work sensibly when the user does so.

At times, not all of these variables may be implemented in the current
release of Autoconf and/or Automake; but as of Autoconf 2.60, we
believe all of them are. When any are missing, the descriptions here
serve as specifications for what Autoconf will implement. As a
programmer, you can either use a development version of Autoconf or
avoid using these variables until a stable release is made which
supports them.

7.2.6 Standard Targets for Users

All GNU programs should have the following targets in their Makefiles:

‘all’

Compile the entire program. This should be the default target. This
target need not rebuild any documentation files; Info files should
normally be included in the distribution, and DVI (and other
documentation format) files should be made only when explicitly asked
for.

By default, the Make rules should compile and link with ‘-g’, so
that executable programs have debugging symbols. Otherwise, you are
essentially helpless in the face of a crash, and it is often far from
easy to reproduce with a fresh build.

‘install’

Compile the program and copy the executables, libraries, and so on to
the file names where they should reside for actual use. If there is a
simple test to verify that a program is properly installed, this target
should run that test.

Do not strip executables when installing them. This helps eventual
debugging that may be needed later, and nowadays disk space is cheap
and dynamic loaders typically ensure debug sections are not loaded during
normal execution. Users that need stripped binaries may invoke the
install-strip target to do that.

If possible, write the install target rule so that it does not
modify anything in the directory where the program was built, provided
‘make all’ has just been done. This is convenient for building the
program under one user name and installing it under another.

The commands should create all the directories in which files are to be
installed, if they don’t already exist. This includes the directories
specified as the values of the variables prefix and
exec_prefix, as well as all subdirectories that are needed.
One way to do this is by means of an installdirs target
as described below.

Use ‘-’ before any command for installing a man page, so that
make will ignore any errors. This is in case there are systems
that don’t have the Unix man page documentation system installed.

The way to install Info files is to copy them into $(infodir)
with $(INSTALL_DATA) (see Command Variables), and then run
the install-info program if it is present. install-info
is a program that edits the Info dir file to add or update the
menu entry for the given Info file; it is part of the Texinfo package.

Here is a sample rule to install an Info file that also tries to
handle some additional situations, such as install-info not
being present.

do-install-info: foo.info installdirs
$(NORMAL_INSTALL)
# Prefer an info file in . to one in srcdir.
if test -f foo.info; then d=.; \
else d="$(srcdir)"; fi; \
$(INSTALL_DATA) $$d/foo.info \
"$(DESTDIR)$(infodir)/foo.info"
# Run install-info only if it exists.
# Use 'if' instead of just prepending '-' to the
# line so we notice real errors from install-info.
# Use '$(SHELL) -c' because some shells do not
# fail gracefully when there is an unknown command.
$(POST_INSTALL)
if $(SHELL) -c 'install-info --version' \
>/dev/null 2>&1; then \
install-info --dir-file="$(DESTDIR)$(infodir)/dir" \
"$(DESTDIR)$(infodir)/foo.info"; \
else true; fi

When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands. See Install Command Categories.

‘install-html’

‘install-dvi’

‘install-pdf’

‘install-ps’

These targets install documentation in formats other than Info;
they’re intended to be called explicitly by the person installing the
package, if that format is desired. GNU prefers Info files, so these
must be installed by the install target.

When you have many documentation files to install, we recommend that
you avoid collisions and clutter by arranging for these targets to
install in subdirectories of the appropriate installation directory,
such as htmldir. As one example, if your package has multiple
manuals, and you wish to install HTML documentation with many files
(such as the “split” mode output by makeinfo --html), you’ll
certainly want to use subdirectories, or two nodes with the same name
in different manuals will overwrite each other.

Please make these install-format targets invoke the
commands for the format target, for example, by making
format a dependency.

‘uninstall’

Delete all the installed files—the copies that the ‘install’
and ‘install-*’ targets create.

This rule should not modify the directories where compilation is done,
only the directories where files are installed.

The uninstallation commands are divided into three categories, just like
the installation commands. See Install Command Categories.

‘install-strip’

Like install, but strip the executable files while installing
them. In simple cases, this target can use the install target in
a simple way:

But if the package installs scripts as well as real executables, the
install-strip target can’t just refer to the install
target; it has to strip the executables but not the scripts.

install-strip should not strip the executables in the build
directory which are being copied for installation. It should only strip
the copies that are installed.

Normally we do not recommend stripping an executable unless you are sure
the program has no bugs. However, it can be reasonable to install a
stripped executable for actual execution while saving the unstripped
executable elsewhere in case there is a bug.

‘clean’

Delete all files in the current directory that are normally created by
building the program. Also delete files in other directories if they
are created by this makefile. However, don’t delete the files that
record the configuration. Also preserve files that could be made by
building, but normally aren’t because the distribution comes with
them. There is no need to delete parent directories that were created
with ‘mkdir -p’, since they could have existed anyway.

Delete .dvi files here if they are not part of the distribution.

‘distclean’

Delete all files in the current directory (or created by this
makefile) that are created by configuring or building the program. If
you have unpacked the source and built the program without creating
any other files, ‘make distclean’ should leave only the files
that were in the distribution. However, there is no need to delete
parent directories that were created with ‘mkdir -p’, since they
could have existed anyway.

‘mostlyclean’

Like ‘clean’, but may refrain from deleting a few files that people
normally don’t want to recompile. For example, the ‘mostlyclean’
target for GCC does not delete libgcc.a, because recompiling it
is rarely necessary and takes a lot of time.

‘maintainer-clean’

Delete almost everything that can be reconstructed with this Makefile.
This typically includes everything deleted by distclean, plus
more: C source files produced by Bison, tags tables, Info files, and
so on.

The reason we say “almost everything” is that running the command
‘make maintainer-clean’ should not delete configure even
if configure can be remade using a rule in the Makefile. More
generally, ‘make maintainer-clean’ should not delete anything
that needs to exist in order to run configure and then begin to
build the program. Also, there is no need to delete parent
directories that were created with ‘mkdir -p’, since they could
have existed anyway. These are the only exceptions;
maintainer-clean should delete everything else that can be
rebuilt.

The ‘maintainer-clean’ target is intended to be used by a maintainer of
the package, not by ordinary users. You may need special tools to
reconstruct some of the files that ‘make maintainer-clean’ deletes.
Since these files are normally included in the distribution, we don’t
take care to make them easy to reconstruct. If you find you need to
unpack the full distribution again, don’t blame us.

To help make users aware of this, the commands for the special
maintainer-clean target should start with these two:

@echo 'This command is intended for maintainers to use; it'
@echo 'deletes files that may need special tools to rebuild.'

‘TAGS’

Update a tags table for this program.

‘info’

Generate any Info files needed. The best way to write the rules is as
follows:

You must define the variable MAKEINFO in the Makefile. It should
run the makeinfo program, which is part of the Texinfo
distribution.

Normally a GNU distribution comes with Info files, and that means the
Info files are present in the source directory. Therefore, the Make
rule for an info file should update it in the source directory. When
users build the package, ordinarily Make will not update the Info files
because they will already be up to date.

‘dvi’

‘html’

‘pdf’

‘ps’

Generate documentation files in the given format. These targets
should always exist, but any or all can be a no-op if the given output
format cannot be generated. These targets should not be dependencies
of the all target; the user must manually invoke them.

You must define the variable TEXI2DVI in the Makefile. It
should run the program texi2dvi, which is part of the Texinfo
distribution. (texi2dvi uses TeX to do the real work of
formatting. TeX is not distributed with Texinfo.) Alternatively,
write only the dependencies, and allow GNU make to provide the
command.

Again, you would define the variable TEXI2HTML in the Makefile;
for example, it might run makeinfo --no-split --html
(makeinfo is part of the Texinfo distribution).

‘dist’

Create a distribution tar file for this program. The tar file should be
set up so that the file names in the tar file start with a subdirectory
name which is the name of the package it is a distribution for. This
name can include the version number.

For example, the distribution tar file of GCC version 1.40 unpacks into
a subdirectory named gcc-1.40.

The easiest way to do this is to create a subdirectory appropriately
named, use ln or cp to install the proper files in it, and
then tar that subdirectory.

Compress the tar file with gzip. For example, the actual
distribution file for GCC version 1.40 is called gcc-1.40.tar.gz.
It is ok to support other free compression formats as well.

The dist target should explicitly depend on all non-source files
that are in the distribution, to make sure they are up to date in the
distribution.
See Making Releases.

‘check’

Perform self-tests (if any). The user must build the program before
running the tests, but need not install the program; you should write
the self-tests so that they work when the program is built but not
installed.

The following targets are suggested as conventional names, for programs
in which they are useful.

installcheck

Perform installation tests (if any). The user must build and install
the program before running the tests. You should not assume that
$(bindir) is in the search path.

installdirs

It’s useful to add a target named ‘installdirs’ to create the
directories where files are installed, and their parent directories.
There is a script called mkinstalldirs which is convenient for
this; you can find it in the Gnulib package.
You can use a rule like this:

7.2.7 Install Command Categories

When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands.

Normal commands move files into their proper places, and set their
modes. They may not alter any files except the ones that come entirely
from the package they belong to.

Pre-installation and post-installation commands may alter other files;
in particular, they can edit global configuration files or data bases.

Pre-installation commands are typically executed before the normal
commands, and post-installation commands are typically run after the
normal commands.

The most common use for a post-installation command is to run
install-info. This cannot be done with a normal command, since
it alters a file (the Info directory) which does not come entirely and
solely from the package being installed. It is a post-installation
command because it needs to be done after the normal command which
installs the package’s Info files.

Most programs don’t need any pre-installation commands, but we have the
feature just in case it is needed.

To classify the commands in the install rule into these three
categories, insert category lines among them. A category line
specifies the category for the commands that follow.

A category line consists of a tab and a reference to a special Make
variable, plus an optional comment at the end. There are three
variables you can use, one for each category; the variable name
specifies the category. Category lines are no-ops in ordinary execution
because these three Make variables are normally undefined (and you
should not define them in the makefile).

Here are the three possible category lines, each with a comment that
explains what it means:

If you don’t use a category line at the beginning of the install
rule, all the commands are classified as normal until the first category
line. If you don’t use any category lines, all the commands are
classified as normal.

Typically, a pre-uninstall command would be used for deleting entries
from the Info directory.

If the install or uninstall target has any dependencies
which act as subroutines of installation, then you should start
each dependency’s commands with a category line, and start the
main target’s commands with a category line also. This way, you can
ensure that each command is placed in the right category regardless of
which of the dependencies actually run.

Pre-installation and post-installation commands should not run any
programs except for these:

The reason for distinguishing the commands in this way is for the sake
of making binary packages. Typically a binary package contains all the
executables and other files that need to be installed, and has its own
method of installing them—so it does not need to run the normal
installation commands. But installing the binary package does need to
execute the pre-installation and post-installation commands.

Programs to build binary packages work by extracting the
pre-installation and post-installation commands. Here is one way of
extracting the pre-installation commands (the -s option to
make is needed to silence messages about entering
subdirectories):

7.3 Making Releases

You should identify each release with a pair of version numbers, a
major version and a minor. We have no objection to using more than
two numbers, but it is very unlikely that you really need them.

Package the distribution of Foo version 69.96 up in a gzipped tar
file with the name foo-69.96.tar.gz. It should unpack into a
subdirectory named foo-69.96.

Building and installing the program should never modify any of the files
contained in the distribution. This means that all the files that form
part of the program in any way must be classified into source
files and non-source files. Source files are written by humans
and never changed automatically; non-source files are produced from
source files by programs under the control of the Makefile.

The distribution should contain a file named README with a
general overview of the package:

the name of the package;

the version number of the package, or refer to where in the
package the version can be found;

a general description of what the package does;

a reference to the file INSTALL, which
should in turn contain an explanation of the installation procedure;

a brief explanation of any unusual top-level directories or
files, or other hints for readers to find their way around the source;

a reference to the file which contains the copying conditions.
The GNU GPL, if used, should be in a file called COPYING. If
the GNU LGPL is used, it should be in a file called
COPYING.LESSER.

Naturally, all the source files must be in the distribution. It is
okay to include non-source files in the distribution along with the
source files they are generated from, provided they are up-to-date
with the source they are made from, and machine-independent, so that
normal building of the distribution will never modify them. We
commonly include non-source files produced by Autoconf, Automake,
Bison, flex, TeX, and makeinfo; this helps avoid
unnecessary dependencies between our distributions, so that users can
install whichever versions of whichever packages they like. Do not
induce new dependencies on other software lightly.

Non-source files that might actually be modified by building and
installing the program should never be included in the
distribution. So if you do distribute non-source files, always make
sure they are up to date when you make a new distribution.

Make sure that all the files in the distribution are world-readable, and
that directories are world-readable and world-searchable (octal mode 755).
We used to recommend that all directories in the distribution also be
world-writable (octal mode 777), because ancient versions of tar
would otherwise not cope when extracting the archive as an unprivileged
user. That can easily lead to security issues when creating the archive,
however, so now we recommend against that.

Don’t include any symbolic links in the distribution itself. If the tar
file contains symbolic links, then people cannot even unpack it on
systems that don’t support symbolic links. Also, don’t use multiple
names for one file in different directories, because certain file
systems cannot handle this and that prevents unpacking the
distribution.

Try to make sure that all the file names will be unique on MS-DOS. A
name on MS-DOS consists of up to 8 characters, optionally followed by a
period and up to three characters. MS-DOS will truncate extra
characters both before and after the period. Thus,
foobarhacker.c and foobarhacker.o are not ambiguous; they
are truncated to foobarha.c and foobarha.o, which are
distinct.

Include in your distribution a copy of the texinfo.tex you used
to test print any *.texinfo or *.texi files.

Likewise, if your program uses small GNU software packages like regex,
getopt, obstack, or termcap, include them in the distribution file.
Leaving them out would make the distribution file a little smaller at
the expense of possible inconvenience to a user who doesn’t know what
other files to get.

8 References to Non-Free Software and Documentation

A GNU program should not recommend, promote, or grant legitimacy to
the use of any non-free program. Proprietary software is a social and
ethical problem, and our aim is to put an end to that problem. We
can’t stop some people from writing proprietary programs, or stop
other people from using them, but we can and should refuse to
advertise them to new potential customers, or to give the public the
idea that their existence is ethical.

A list of important licenses and whether they qualify as free is in
http://www.gnu.org/licenses/license-list.html. If it is not
clear whether a license qualifies as free, please ask the GNU Project
by writing to licensing@gnu.org. We will answer, and if the
license is an important one, we will add it to the list.

When a non-free program or system is well known, you can mention it in
passing—that is harmless, since users who might want to use it
probably already know about it. For instance, it is fine to explain
how to build your package on top of some widely used non-free
operating system, or how to use it together with some widely used
non-free program.

However, you should give only the necessary information to help those
who already use the non-free program to use your program with
it—don’t give, or refer to, any further information about the
proprietary program, and don’t imply that the proprietary program
enhances your program, or that its existence is in any way a good
thing. The goal should be that people already using the proprietary
program will get the advice they need about how to use your free
program with it, while people who don’t already use the proprietary
program will not see anything likely to lead them to take an interest
in it.

If a non-free program or system is obscure in your program’s domain,
your program should not mention or support it at all, since doing so
would tend to popularize the non-free program more than it popularizes
your program. (You cannot hope to find many additional users for your
program among the users of Foobar, if the existence of Foobar is not
generally known among people who might want to use your program.)

Sometimes a program is free software in itself but depends on a
non-free platform in order to run. For instance, many Java programs
depend on some non-free Java libraries. To recommend or promote such
a program is to promote the other programs it needs. This is why we
are careful about listing Java programs in the Free Software
Directory: we don’t want to promote the non-free Java libraries.

We hope this particular problem with Java will be gone by and by, as
we replace the remaining non-free standard Java libraries with free
software, but the general principle will remain the same: don’t
recommend, promote or legitimize programs that depend on non-free
software to run.

Some free programs strongly encourage the use of non-free software. A
typical example is mplayer. It is free software in itself,
and the free code can handle some kinds of files. However,
mplayer recommends use of non-free codecs for other kinds of
files, and users that install mplayer are very likely to
install those codecs along with it. To recommend mplayer
is, in effect, to promote use of the non-free codecs.

Thus, you should not recommend programs that strongly encourage the
use of non-free software. This is why we do not list
mplayer in the Free Software Directory.

A GNU package should not refer the user to any non-free documentation
for free software. Free documentation that can be included in free
operating systems is essential for completing the GNU system, or any
free operating system, so encouraging it is a priority; to recommend
use of documentation that we are not allowed to include undermines the
impetus for the community to produce documentation that we can
include. So GNU packages should never recommend non-free
documentation.

By contrast, it is ok to refer to journal articles and textbooks in
the comments of a program for explanation of how it functions, even
though they are non-free. This is because we don’t include such
things in the GNU system even if they are free—they are outside the
scope of what a software distribution needs to include.

Referring to a web site that describes or recommends a non-free
program is promoting that program, so please do not make links to (or
mention by name) web sites that contain such material. This policy is
relevant particularly for the web pages for a GNU package.

Following links from nearly any web site can lead eventually to
non-free software; this is inherent in the nature of the web. So it
makes no sense to criticize a site for having such links. As long as
the site does not itself recommend a non-free program, there is no
need to consider the question of the sites that it links to for other
reasons.

Thus, for example, you should not refer to AT&T’s web site if that
recommends AT&T’s non-free software packages; you should not refer to
a site that links to AT&T’s site presenting it as a place to get some
non-free program, because that link recommends and legitimizes the
non-free program. However, that a site contains a link to AT&T’s web
site for some other purpose (such as long-distance telephone service)
is not an objection against it.

The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of “copyleft”, which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.

APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein. The “Document”, below,
refers to any such manual or work. Any member of the public is a
licensee, and is addressed as “you”. You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.

A “Modified Version” of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A “Secondary Section” is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document’s overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The “Invariant Sections” are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.

The “Cover Texts” are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.

A “Transparent” copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text. A copy that is not “Transparent” is called “Opaque”.

Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input
format, SGML or XML using a publicly available
DTD, and standard-conforming simple HTML,
PostScript or PDF designed for human modification. Examples
of transparent image formats include PNG, XCF and
JPG. Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, SGML or
XML for which the DTD and/or processing tools are
not generally available, and the machine-generated HTML,
PostScript or PDF produced by some word processors for
output purposes only.

The “Title Page” means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
formats which do not have any title page as such, “Title Page” means
the text near the most prominent appearance of the work’s title,
preceding the beginning of the body of the text.

The “publisher” means any person or entity that distributes copies
of the Document to the public.

A section “Entitled XYZ” means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as “Acknowledgements”,
“Dedications”, “Endorsements”, or “History”.) To “Preserve the Title”
of such a section when you modify the Document means that it remains a
section “Entitled XYZ” according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document’s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify
you as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version
if the original publisher of that version gives permission.

List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

State on the Title page the name of the publisher of the
Modified Version, as the publisher.

Preserve all the copyright notices of the Document.

Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document’s license notice.

Include an unaltered copy of this License.

Preserve the section Entitled “History”, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page. If
there is no section Entitled “History” in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on. These may be placed in the “History” section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section titles.

Delete any section Entitled “Endorsements”. Such a section
may not be included in the Modified Version.

Do not retitle any existing section to be Entitled “Endorsements” or
to conflict in title with any Invariant Section.

Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements”,
and any sections Entitled “Dedications”. You must delete all
sections Entitled “Endorsements.”

COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document’s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy’s public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

RELICENSING

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
“Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC
site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

“Incorporate” means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (C) yearyour name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with…Texts.” line with this:

with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.